Persistent tendinitis limited Hargreaves to six minutes of playing time at United after having surgery on both knees during the 2008-09 season.
The 30-year-old midfielder is trying to kickstart his career at Manchester City after being released by United at the end of last season.
And Hargreaves, who scored a goal on his City debut on Wednesday, claims United mishandled his rehabilitation after the 2007-08 season, which ended with the club winning the Champions League.
“I received some injections but my tendon was never the same,” the former Bayern Munich player told British newspapers after City beat Birmingham 2-0 in the League Cup. “After the injections, I tried to get back on my feet and they said my tendon was good, but it felt like I was made out of glass. That obviously had a huge impact. With my tendon injury, I’ve had to be a guinea pig for a lot of these treatments. It’s difficult.”
The Canadian-born player, who competed for England at the 2006 World Cup, said he was assured by United that there would be no side effects from the painkilling injections.
“That wasn’t the case and if I’d known, I wouldn’t have done it,” he said. “I wish I had not had the injections … it got significantly worse after that through the injections I had. After that my tendon was never the same.”
United manager Alex Ferguson defended his medical staff on Friday, while saying he did not know the specifics of the treatment.
“We’re trying to analyze all the stuff he has said and there will be a statement later to address that,” Ferguson said. “As far as I am concerned my medical staff is one of the main reasons why we have been so successful over the last few years.
“Do you think we would be successful if they were not 100 percent brilliant? Our doctor and his five physios are fantastic. That is my opinion and that is more important than what anyone else has to say about it.”
City manager Roberto Mancini said Hargreaves could be a regular member of his squad within a month if his present progress continues.
“He is pain-free,” Mancini said. “There are no problems, apart from a bit of tiredness. He has been out for two years, which is a long period. He has worked hard for the last six months in Canada. And I hope it has solved all his problems.”