Lionel Messi looks likely to miss Argentina's most important match of the opening phase of the Copa America against title holders Chile at Santa Clara, California on Monday.
Messi has not trained with his team since he suffered a back injury in a warm-up friendly nine days ago, a 1-0 win over Honduras in Argentina.
The Argentina captain, four times world player of the year, has been working separately and may not even make it onto the field at Levi's Stadium, according to reports on Sunday from Argentine media covering the national team's base camp at San Jose.
His place will be taken by Nicolas Gaitan, a player who has won 13 caps in six years but who coach Gerardo Martino has identified as his best reserve for Messi without altering tactics.
A left-footer who likes playing on the right and cutting inside in attack, with his cub Benfica's best assists record in the Portuguese league, his game resembles what Messi does best for his country.
"I wouldn't risk him, I don't want to lose him in the first match," defender Marcos Rojo was quoted as saying with the team fearing an aggravation of Messi's injury could put him out for the rest of the tournament.
Chile would be expected to feel some relief at the possible absence of the world's best player but their captain and goalkeeper, Messi's Barcelona team mate Claudio Bravo, said: "I want him to play because he adds a lot to the match."
On Paper, the other Group D matches against Panama and Bolivia look easy for title favorites Argentina and Chile.
Colombia are also fretting on the possible absence of their captain James Rodriguez from their second Group A match against Paraguay at the Rose Bowl in Pasadena on Tuesday after he hurt his shoulder during the 2-0 win over hosts the United States at Santa Clara on Friday.
RANKING THE 50 MOST DOMINANT ATHLETES ALIVE:
50 most dominant athletes alive (BI)
Messi could miss potential group decider against Chile
50. Miesha Tate UFC fighter
Miesha Tate has won five fights in a row, dating back to 2014. Her most recent win was over Holly Holm — who beat Ronda Rousey — via submission. A rematch with Rousey, to whom Tate lost to in 2013, would be intriguing.
(Photo by Christian Petersen/Zuffa LLC/Zuffa LLC via Getty Images)
49. N'Golo Kante
Leicester City center midfielder
Kante was the best player on the best team in England this season, the speedy box-to-box midfielder who unlocked Leicester City's devastating counterattack by defending well and distributing even better. The Foxes' dream season wouldn't have happened without Kante, and now the French international is expected to sign with a bigger club in England.
(Photo by Mustafa Yalcin/Anadolu Agency/Getty Images)
48. Teddy Riner Judoka
Teddy Riner hasn't lost a match since 2010. The French heavyweight has won Olympic gold in 2012, six world championships, including the last three from 2013-15, and the 2015 World Masters.
(Photo credit should read MARTIN BUREAU/AFP/Getty Images)
47. Maya Moore
Minnesota Lynx forward
In a down season, coming off the MVP in 2014, Moore still posted a gaudy stat line of over 20 points per game, 6.7 rebounds, 3.5 assists, and nearly two steals per game. A bounce-back season should put her right back in the MVP mix in 2016.
(Photo by Caroline Yang/NBAE via Getty Images)
46. Conor McGregor UFC fighter
Prior to a March loss to Nate Diaz, Irish featherweight Conor McGregor had won 15 fights in a row, with the last six ending in TKO. His hold on the sport is such that he caused the fighting world to freak out when he faked his retirement over a dispute with UFC.
(Photo by Christian Petersen/Zuffa LLC/Zuffa LLC via Getty Images)
45. Draymond Green
Golden State Warriors forward
Draymond Green is a question for which the NBA has no answer. The 6-foot-6 forward can play and guard all five positions, unlocking endless lineup possibilities for the Warriors. It's not unusual to see Green block a shot on one end, grab the rebound, bring the ball up the court, and dish it to a teammate or get it back to hit a three. He nearly averaged a triple-double in 2015-16.
(Photo by Ezra Shaw/Getty Images)
44. Hope Solo
US Women's National Team goalie
Hope Solo is a stalwart. At last summer's Women's World Cup, she played every minute of every game (and conceded only three goals). At the 2012 London Olympics, she played every minute of every game. Same with the 2011 World Cup, and the 2008 Beijing Olympics. Don't expect anything else this summer in Rio ... even if she says she's reluctant to go because of the Zika virus.
(Photo by Jim Rogash/Getty Images)
43. Steve Smith Australian cricketer
Smith is the best cricketer in the world, plain and simple. Last season he was named the best player in Australia, and later on in 2015 won the Sir Garfield Sobers trophy, given to the best cricketer in the world. You don't need to know much about cricket to appreciate Smith's talent.
(Photo by Ryan Pierse/Getty Images)
42. Lewis Hamilton Formula One driver
The British playboy and Mercedes driver continues to be the class of the Formula One circuit. When he's not driving around Los Angeles in a LaFerrari with Justin Bieber, he's winning World Championships. This year, he'll look for the three-peat, having won the 2014 and 2015 World Formula One Championship.
(Photo by Mark Thompson/Getty Images)
41. Clayton Kershaw Los Angeles Dodgers pitcher
Clayton Kershaw is looking to post his best season in what's already been, arguably, the most dominant six-year run in pitching history. He's currently posting a 1.67 ERA with 88 strikeouts to just four walks in nine starts this season. There isn't a more fearsome arm in MLB.
(Photo by Stephen Dunn/Getty Images)
40. Tom Brady
New England Patriots quarterback
At an age when many players are looking into retirement, Tom Brady keeps chugging along. Brady just posted one of his most efficient seasons in 2015, and regardless of a suspension to start the year, the Patriots figure to be vying for another Super Bowl again in 2016 because of Brady's agelessness.
(Photo by Jim Rogash/Getty Images)
39. Yana Kudryavtseva
Yana Kudryavtseva has been dominant since entering the senior ranks in 2013. The Russian gymnast has three consecutive world championships to her name with 16 world championship medals, 13 of which are gold. She'll be looking for more at the Rio Olympics.
(Photo credit should read THOMAS KIENZLE/AFP/Getty Images)
38. Kevin Durant Oklahoma City Thunder forward
There's only one player in the NBA to average more than 27 points, 45% shooting, and seven rebounds per game combined since 2011 — not LeBron James, but Kevin Durant. Durant finished fourth in MVP voting this season, and yet it still somehow felt like he flew under the radar. The question now is whether he'll be in a Thunder uniform after this season.
(Photo by Andrew D. Bernstein/NBAE via Getty Images)
37. Gwen Jorgensen American triathlete
A former Ernst & Young accountant, Jorgensen is now the best female triathlete in the world at the Olympic distance, and it's not much of a competition. Not only has she won 13 of the last 14 triathlons she's raced, she's also earned gold at each of the last two World Championships. In 2012 at the London Olympics, Jorgensen suffered a flat tire on her bike and finished a disappointing 38th. She's the heavy favorite to win gold at Rio this summer.
(Photo by Buda Mendes/Getty Images)
36. Mike Trout Los Angeles Angels centerfielder
Since 2012, Mike Trout has won an American League MVP and finished second place in MVP voting three times. What did you accomplish at 24?
(Photo by Robert Huskey/Angels Baseball LP/Getty Images)
35. Alex Ovechkin Washington Capitals wing
Alex Ovechkin remains the NHL's most fearsome scorer. In 2015-16, Ovechkin led the Capitals to the best record in the NHL and the most points, all while leading the league in goals (for the fourth year in a row) and total shots, a skill unto itself.
(Photo by Len Redkoles/NHLI via Getty Images)
34. Mo Farah British middle-distance runner
Farah is the most decorated runner in British history, which is no easy feat. At the 2015 Worlds he won both the 5,000-meter and 10,000-meter events. Prior, he also set the indoor record for the two-mile (8:03.04) and broke the European record for the half marathon (59:32.20). He may be 33, but he's showing no signs of slowing down. Expect big things at this summer's Olympics.
(Photo credit ANDY BUCHANAN/AFP/Getty Images)
33. Kawhi Leonard San Antonio Spurs forward
Leonard has always been a great defensive player, but this past season saw the Spurs young gun take a major leap offensively. He was the best player on a 67-win Spurs team, finished second in MVP voting, and won his second consecutive Defensive Player of the Year award. The best two-way player in the NBA is the face of the future Spurs.
(Photo by J Pat Carter/Getty Images)
32. Michael Phelps Swimmer
The most decorated Olympian of all time came out of retirement in 2015 and picked up where he left off. Phelps swam at the U.S. National Championships and swept all three of his events and swam the year's fastest times for each. At the Arena Pro Swim event in April, Phelps finished first in the 200 IM and 200m butterfly. He looks like he'll dominate again at the 2016 Olympics.
(Photo by Chris Coduto/Getty Images)
31. Lizzie Armistead British cyclist
She's having a perfect 2016 (four wins in four races), but Lizzie Armistead's run of dominance in the women's professional cycling circuit dates all the way back to the 2012 Olympics, where she picked up a silver medal in the road race not long after switching over from the track. In 2015, Armistead raced in 11 events and won nine of them (she finished first and third in the other two). A powerful sprinter and aggressive climber, Armistead is an all-around force.
(Photo by Ben Duffy/adidas/Handout/Getty Images)
30. Luis Suarez
F.C. Barcelona striker
What a prolific season it was for the Uruguayan No. 9 at Camp Nou. Forty goals in La Liga for Suarez this season is simply mind-boggling, as were the 14 goals he tallied in the final five league matches. Not since Diego Forlan in 2009 has a player not named Cristiano Ronaldo or Lionel Messi led La Liga in goals, but this year it belonged to Suarez. He promises to terrorize opponents this summer at the Copa America.
(Photo credit should read JORGE GUERRERO/AFP/Getty Images)
29. Rob Gronkowski New England Patriots tight end
It's unusual for perhaps the NFL's best pass-catcher to not be a wide receiver, but that may be the case with Rob Gronkowski. The 6-foot-6 tight end is a matchup conundrum — too big to slot cornerbacks onto, too fast for bigger defenders. Tom Brady, a quarterback known for turning anyone into a valuable receiver, said Gronkowski makes his life easier.
(Photo by Jim Rogash/Getty Images)
28. Monica Abbott Softball pitcher
Monica Abbott is so dominant that in a league that squeezes 18 players under a $150,000 salary cap, she just earned the first $1 million contract in National Pro Fastpitch (NPF). The cost is worth it for a pitcher who posted a 0.31 ERA, 0.59 WHIP, and 149 strikeouts in over 90 innings while leading her team to a title last season.
(Photo by Joe Rimkus Jr./Miami Herald/MCT via Getty Images)
27. Ashton Eaton U.S. decathlete
Eaton broke his own world record in the decathlon last summer, and come August in Rio, he will look to break it yet again. There's no event quite like the decathlon — to win gold medals and world championships (as Eaton as), you need to be a well-rounded athlete capable of jumps, sprints, hurdles, throws, and more. In terms of pure athleticism, Eaton rivals anyone on this list.
(Photo credit DON EMMERT/AFP/Getty Images)
26. Genzebe Dibaba
Ethiopian distance star Genzebe Dibaba owns five world records after breaking the 26-year-old indoor mile record by four seconds in February 2016 at the IAAF World Indoor Tour. She also owns the 1,500 meters, having set both the indoor and outdoor world records in the event, and will look to become the first woman to sweep both races in Rio this summer.
(Photo credit PEDRO UGARTE/AFP/Getty Images)
25. Carli Lloyd U.S. Women's National Team midfielder
Lloyd may be in the twilight of her career, but her goal in the final of last year's World Cup was the single greatest goal in American soccer history. Does anyone even remember that she had a hat-trick in the first half game against Japan, or just the half-field missile? Either way, Lloyd is at the pinnacle of women's soccer and, whenever she retires, will be hard to replace.
The undefeated middleweight champion of the world, Golovkin is an old-school brawler. Not only does he boast the highest knockout percentage in middleweight history (91%), he's also never in his life, as an amateur or professional over 375 fights, been knocked down. A fight against Canelo Alvarez is rumored, and it would be the biggest in the sport since Mayweather vs. Pacquiao.
(Photo by Harry How/Getty Images)
23. Elena Delle Donne Chicago Sky forward
Elena Delle Donne's third season was the best of her short career as she led the league in scoring with over 23 points per game on 46% shooting, to go with over eight rebounds, a steal, and two blocks per game en route to winning MVP.
(Photo by Nathaniel S. Butler/NBAE via Getty Images)
22. Jordan Spieth Professional golfer
It's been an up-and-down 2016 for Spieth, but that doesn't take away from a fantastic 2015 that saw him win the Masters and the U.S. Open, and finish the year as the world no. 1. This year, a collapse at the Masters and some missed cuts have set him back — all the way to world no. 2. In other words, Spieth is the future of golf.
(Photo by Tom Pennington/Getty Images)
21. Bryce Harper Washington Nationals right fielder
Injuries forced Bryce Harper's career to get off to a slow roll, but now that he's healthy, he looks like the mega-star MLB has been missing. In 2015, Harper hit .330 (fourth in MLB), with a .460 OBP (first), .649 SLG (first), 1.109 OPS (first), 9.0 WAR (first), to go with 42 home runs (third). He's already getting the Barry Bonds treatment at the plate.
(Photo by Patrick McDermott/Washington Nationals/Getty Images)
20. Jason Day Golfer
Jason Day has slowly climbed the ladder in the PGA Tour to finally reach the world No. 1 ranking in 2016. Since a ninth-place finish at the U.S. Open last June, Day has six first-place finishes, including winning the 2015 PGA Championship. With Spieth's choppy start to 2016 and Rory McIlroy's struggles on the green, Day seems like the most stable force in golf.
(Photo by Richard Heathcote/Getty Images)
19. Usain Bolt Jamaican sprinter
Even though he doesn't compete as frequently as many of his competitors, when Bolt takes the blocks, a win feels like a foregone conclusion. Last summer's world championship saw Bolt win the 100m, 200m, and 4 x 100m — and he was having the worst season since before he won gold in Beijing in 2008. Is Bolt worried about the conditions in Brazil? Of course not. He says he's too fast to get bitten by mosquitoes.
(Photo by Mustafa Yalcin/Anadolu Agency/Getty Images)
18. Peter Sagan Slovak cyclist
Sagan won the green jersey for points at last summer's Tour de France, and followed it up with an even more impressive performance at the World Road Race Championship in Richmond, which he won in crafty fashion — surging on an uphill cobblestone section and never looking back. His bike — which is customized to reflect his world championship — is the coolest in the game, too.
(Photo by Bryn Lennon/Getty Images)
17. Patrick Kane Chicago Blackhawks winger
He's only 27, but Kane has already won three Stanley Cups over the course of his career, and a Conn Smythe Trophy. This season, he led the NHL in points, which won him the Art Ross Trophy. His handles and prowess around the net are second to none in hockey, even if the Hawks bowed out earlier than expected in this year's playoffs.
(Brian Cassella/Chicago Tribune/TNS via Getty Images)
16. Cam Newton Carolina Panthers quarterback
Cam Newton isn't the NFL's purest or best quarterback, but he embodies what every team wants at the position. He can make any throw, he's developed into a strong pocket passer, and when he flees the pocket, he's nearly impossible to bring down. He's a physical specimen that captained a 15-1 team and is quickly improving every one of his weaknesses.
(Photo by Grant Halverson/Getty Images)
15. Lydia Ko Professional golfer
Ko is the world's best female golfer, and well on her way to being one of the best ever. She's just 19, making her the youngest world no. 1 ever, and she already has 12 LPGA victories under her belt, including two majors. In September 2015, she shot a 63 — the best score on a Sunday in the history of the LPGA. Expect big things from Ko this season, and for the next couple of decades or so.
(Photo by Robert Laberge/Getty Images)
14. Marcel Hirscher Skier
Marcel Hirscher remains the No. 1-ranked alpine skier in the world after a dominant 2015 that saw him win his fifth World Cup title, with eight more victories and ten other podium finishes, to go with his four world championships.
(Photo credit should read FABRICE COFFRINI/AFP/Getty Images)
13. Cristiano Ronaldo Real Madrid forward
It often feels like Ronaldo plays La Liga's second fiddle to Lionel Messi, but Ronaldo continues to be one of the best three players in the world. He scored a cool 35 goals in 35 matches for Real Madrid this season, and since his arrival in 2009 he has scored 260 goals. Later this month he'll have a shot at his second Champions League title with Real Madrid.
(Photo credit MIGUEL RIOPA/AFP/Getty Images)
12. Jake Arrieta Chicago Cubs pitcher
The reigning N.L. Cy Young winner has been so dominant this season that people are accusing him of taking steroids. He's 7-0 in eight starts, with a 1.29 ERA and a 0.84 WHIP. Opposing batters are hitting just .153. How well the Cubs do come the playoffs will likely come down to how well he pitches. So far, Chicago fans have to be feeling pretty good.
(Photo by Joe Sargent/Getty Images)
11. Kohei Uchimura Gymnast
After winning his sixth consecutive world championship in 2015, USA Today's Nancy Armour called Kohei Uchimura the "greatest gymnast ever," noting the last time he finished second in the World Championships or Olympics, Aaron Rodgers hadn't ever started an NFL game. The Japanese gymnast has also won eight consecutive NHK Cups and gold in the all-around in the 2012 Olympics. He will be looking for more in Rio this summer.
(Photo by Ian MacNicol/Getty images)
10. Von Miller Denver Broncos defensive end
It's rare to see a defensive player carry a team through the postseason, but that's exactly what Von Miller did in helping the Denver Broncos win the 2015 Super Bowl. The Super Bowl MVP had 2.5 sacks, two forced fumbles, and directly led to two Broncos' scores in the NFL's biggest game. There's not a more fearsome pass-rusher in football right now.
(Photo by Patrick Smith/Getty Images)
9. Claressa Shields American middleweight boxer
Shields burst onto the scene as a teenager at the 2012 London Olympics, winning gold in dominating fashion. She hasn't lost since. She's 68-1 with 18 KOs, and won the 2014 World Championship and 2015 PanAm Games, where she moved up to the light-heavyweight division. She's expected to cruise to gold in Rio.
(Photo credit ALBERTO PIZZOLI/AFP/GettyImages)
8. LeBron James
Cleveland Cavaliers forward
There's nobody else in the NBA who can control a game in the way LeBron James can. Whether it's shooting, posting up, attacking the basket, facilitating, defending — James can pick his spots whenever and wherever he chooses. Coupled with seemingly everlasting physical superiority, James is still in category unto himself when it comes to dictating a game.
(Photo by Andy Lyons/Getty Images)
7. Serena Williams American tennis player
Sheer power alone makes it sometimes feel as though Serena Williams is playing an entirely different sport than her competitors. Although she lost in the Australian Open final, and missed out on a calendar slam last season when she fell in the U.S. Open semis, Williams is still the best female tennis player alive and soon to be the most decorated. With 21 majors, she's one away from matching Stefi Graff with the most all-time in the modern era. It seems inevitable that she'll pass Graff.
(Photo credit should read TIZIANA FABI/AFP/Getty Images)
6. Lionel Messi F.C. Barcelona forward
Messi is the best player in the world, full stop. His quickness, foot-skills, and vision around the field make his every touch must-watch TV, as he's capable of changing the entire flow of a game in a sudden whirl. The accolades are all there (he's won a record five Ballon d'Or trophies) but no matter how many trophies he wins and titles he earns, none seem to quite capture the magic of watching Messi move on the field. "Poetry in motion" is pretty close.
(Photo credit LLUIS GENE/AFP/Getty Images)
5. Simone Biles American gymnast
Undoubtedly the shortest athlete on our list, what the 4-foot-9 Biles lacks in height she makes up for in thorough perfection on the mats, vault, and beam. She's already the most decorated gymnast in American history, and the only gymnast ever to win three-straight World Championships in the all-around competition. This summer, when she is introduced to the American public at her first Olympics, she's going to be a bona fide superstar. Expect to see her on every talk show soon, and more importantly, expect her to absolutely destroy her competition.
(Photo by Ezra Shaw/Getty Images)
4. Breanna Stewart
Seattle Storm forward
Stewart was the best player on arguably the most dominant team in all of sports: the UConn Huskies, who have won 90 games in a row. UConn won the NCAA Championship the last four years while she led them in scoring and rebounds per game for the past three. She was miles ahead of any other women's college-basketball player, and as the No. 1 pick in the WNBA draft, is expected to continue her dominance in the pros.
(Photo by Juan Ocampo/NBAE via Getty Images)
3. Stephen Curry
Golden State Warriors guard
Stephen Curry has wrestled away the title of best basketball player in the world from LeBron James. His résumé is enough to justify it: two-time MVP, first unanimous MVP, scoring champion, NBA champion, best player on a team that won more regular season games than any team in NBA history. And that's just the last two years. Perhaps more importantly, Curry is a game-bending force that's changing the way offenses and defenses behave across the league. We've never seen anything like him.
(Raj Mehta-USA TODAY Sports)
2. Katie Ledecky
Katie Ledecky is already a force to be reckoned with in the pool, and she's not even of a legal drinking age. She won an Olympic Gold in the 800-meter freestyle at 15 years old, has won nine golds in two World Championships, and owns three current world records. Her world record in the 800-meter freestyle is seven seconds faster than the next competitor, and she now owns the eight fastest times in the history of the event.
(Photo by Kevin C. Cox/Getty Images)
1. Novak Djokovic
Serbian tennis player
What a run Novak Djokovic is on. Last season, the world No. 1 won three majors in convincing fashion and lost in the finals of the French Open, meaning he was just one match shy of the elusive calendar slam. He's so good he sometimes seems almost robotic on court, returning serve better than just about anyone in the history of the sport and grinding out long, improbable rallies with great defense and sudden down-the-line winners.
His 11 majors already put him in conversation with the all-time greats, but if he can win the French this year and finally complete the career slam, that will put him in even more elite company. It will also mean he will have completed a DjokerSlam, for he would hold every major concurrently.
There's simply no doubt about it. Novak Djokovic is the most dominant athlete alive.