According to BBC Sport, Shamil Tarpishchev told the R-Sport news agency that he thought it was "very doubtful" that Sharapova would play future tournaments, adding that she was in a "bad situation."
Tarpishchev's latest comments signal a noticeable departure from his staunch defense of Sharapova back in March, when she first announced the positive test in an unexpected news conference that many thought would be a retirement announcement.
Sharapova said she had continued to take meldonium, even after receiving multiple emails warning that it had been added to the World Anti-Doping Agency's list of banned substances for 2016, because she knew it by the name mildronate.
After the International Tennis Federation provisionally suspended her on March 12, the five-time Grand Slam winner said she was "determined to play tennis again."
The severity of Sharapova's punishment is still up in the air. The 29-year-old could be suspended for as many as four years, which would most likely mark the end of her career. But the BBC reports that a six- or 12-month suspension is more likely because WADA acknowledged in April that it was unsure how long meldonium stayed in an athlete's system.
Russian tennis boss says Maria Sharapova's return to tennis is 'very doubtful'
LOS ANGELES, CA - MARCH 07: Tennis player Maria Sharapova reacts as she addresses the media regarding a failed drug test at The LA Hotel Downtown on March 7, 2016 in Los Angeles, California. Sharapova, a five-time major champion, is currently the 7th ranked player on the WTA tour. Sharapova, withdrew from this weekÂs BNP Paribas Open at Indian Wells due to injury. (Photo by Kevork Djansezian/Getty Images)
Maria Sharapova of Russia gets set for a serve from Monica Seles during their match Thursday, March 7, 2002, in Indian Wells, Calif. Seles won the match 6-0, 6-2. (AP Photo/Mark J. Terrill)
Maria Sharapova of Russia reacts after missing a shot during her semi-final match against Monica Seles of the United States in the Hong Kong Ladies Challenge tennis tournament in Hong Kong Friday, Jan. 3, 2003. Seles won 6-3, 6-0. (AP Photo/Vincent Yu)
Russia's Maria Sharapova returns to Yugoslavia's Jelena Dokic, during their Women's Singles, third round match on the Number One Court, on the sixth day of the All England Lawn Tennis Championships at Wimbledon, Saturday, June 28, 2003. Sharapova won the match 6-4, 6-4.(AP Photo/Dave Caulkin)
Russian tennis player Maria Sharapova, center, smiles during a demonstration session at Hong Kong's Victoria Tennis Court Sunday, Jan. 4, 2004. Sharapova will have friendly match with Venus Williams and Chanda Rubin of the United States, and Russia's Elena Dementieva on Jan. 8-10. (AP Photo/Vincent Yu)
Russia's Maria Sharapova reacts after defeating Yulia Beygeizimer of the Ukraine in their Women's Singles, first round match at Wimbledon, Monday June 21, 2004. (AP Photo/Sang Tan) ** EDITORIAL USE ONLY **
Maria Sharapova, of Russia, serves to Amelie Mauresmo (not shown), of France, during their semifinal match at the WTA Championships at Staples Center in Los Angeles, Saturday, Nov. 12, 2005. Mauresmo won, 7-6, 6-3. (AP Photo/Chris Carlson)
Maria Sharapova from Russia serves against Justine Henin-Hardenne from Belgium during her WTA Championships tennis tournament semi-final match in Madrid, Saturday, Nov. 11, 2006. Justine Henin-Hardenne won the match in two sets 6-2, 7-6. (AP Photo/Bernat Armangue)
Russia's Maria Sharapova celebrates winning her match against fellow countrywoman Anna Chakvetadze during the Sony Ericsson Championships match in Madrid, Saturday, Nov. 10, 2007. Sharapova won 6-2, 6-2. (AP Photo/Daniel Ochoa de Olza)
Russia's tennis player Maria Sharapova looks on during a press conference, Saturday Dec. 29, 2007, in Singapore where she will play against compatriot Anna Chakvetadze at an exhibition match on Sunday. (AP Photo/Wong Maye-E)
Maria Sharapova, of Russia, returns the ball against Daniela Hantuchova at the Pacific Life Open tennis tournament in Indian Wells, Calif., Wednesday, March 19, 2008. (AP Photo/Mark Avery)
Tennis Player Maria Sharapova unveils a limited edition Canon PowerShot diamond encrusted camera on Wednesday, Aug. 20, 2008, in New York. (AP Photo/Peter Kramer)
Maria Sharapova of Russia holds the trophy after the final match against Jelena Jankovic of Serbia at the Pan Pacific Open tennis tournament in Tokyo, Japan, Saturday, Oct. 3, 2009. Former No. 1-ranked Sharapova won her first tournament since returning from a 10-month injury layoff when Jankovic was forced to retire in the first set. Jankovic retired with an injured right arm trailing 5-2 in the first set. (AP Photo/Itsuo Inouye)
Russia's Maria Sharapova waves to the public after defeating compatriot Ksenia Pervak during their first round match of the French Open tennis tournament at the Roland Garros stadium in Paris, Tuesday, May 25, 2010. (AP Photo/Laurent Baheux)
Professional tennis player Maria Sharapova watches the New Jersey Nets vs. the Charlotte Bobcats in the second quarter of an NBA basketball game, Monday, April 11, 2011 in Newark, N.J. (AP Photo/Julio Cortez)
Maria Sharapova of Russia reacts during a tennnis match against Samantha Stosur of Australia at the WTA championship in Istanbul, Turkey, Tuesday, Oct. 25, 2011. Sharapova lost to Stosur 6-1, 7-5. (AP Photo/Vadim Ghirda)
Maria Sharapova of Russia, serves to Roberta Vinci of Italy, during their match at the BNP Paribas Open tennis tournament, Tuesday, March 13, 2012, in Indian Wells, Calif. (AP Photo/Mark J. Terrill)
Russia's Maria Sharapova looks up after beating Australian Samantha Stosur 6-7, 7-6, 7-6 during their quarterfinal match at the Porsche tennis Grand Prix in Stuttgart, Germany, Friday, April 27, 2012. (AP Photo/Michael Probst)
Maria Sharapova arrives at the 3.1 Phillip Lim for Target launch event at Spring Studio on Thursday, Sept. 5, 2013 in New York. (Photo by Dario Cantatore/Invision/AP)
Maria Sharapova of the Manila Mavericks autographs large tennis balls of fans following her win over Kristina Mladenovic of the UAE Royals in their IPTL (International Premier Tennis League) Women's Singles match Friday, Nov. 28, 2014 at the Mall of Asia Arena at suburban Pasay city, south of Manila, Philippines. The IPTL, featuring four teams, introduces a new format in tennis and a chance for a championship prize of $1-million dollars. (AP Photo/Bullit Marquez)
Maria Sharapova of Russia celebrates after defeating Flavia Pennetta of Italy during their singles match at the WTA tennis finals in Singapore on Thursday, Oct. 29, 2015. (AP Photo/Wong Maye-E)