Owen Hargreaves has claimed Manchester United put his career on the line by administering the wrong treatment for his chronic injury problems and making a series of mistakes that left him fearing he may never play football again.

Speaking for the first time about his "guinea pig" treatment at Old Trafford, Hargreaves claimed the club's medical staff decided he needed a course of injections for his knee injury. There is no evidence whatsoever that Hargreaves was misdiagnosed or improperly treated, but the midfielder believes the treatment led to his condition deteriorating, leaving him barely able to walk and with a knee that felt "like I was made out of glass".

He claims various specialists had subsequently told him United had prescribed the wrong treatment, adding that his injury "got significantly worse through the injections". A knee specialist later diagnosed an extreme form of tendinitis that led to major surgery and put him out of the game for effectively three years.

"I've had to be a guinea pig for a lot of these treatments," said Hargreaves. "But when you're left to try to make something of a difficult situation, it's not really an option to say: 'Let's sit it out and rest for six months.'"

Hargreaves, now of Manchester City, talked of being put "in a shit position" before making the remarkable claim that the United manager Sir Alex Ferguson selected him for his comeback match against Wolverhampton Wanderers last November even though the midfielder had told the club he was injured.

"I started that game with two muscle injuries but I should never have been in that position to begin with. I was just going to try to get through 45 minutes because that would have been a start. But I didn't even last five. I wasn't surprised. I'd said to them that I had a hamstring problem, which I obviously did. I said: 'I think I've got a bit of a problem here.' I was surprised it didn't go in the warm-up, to be completely honest.

"People at times must look at it from a distance and say: 'Shit, he's made out of glass.' But I don't know many people in the world who could do any sporting event with two muscle tears."

Hargreaves said he felt so unfit he knew he could not even run at full pace. "There were 70,000 people there and, if I had walked away from the game, it would have looked like I didn't want to play. I thought: 'I'll try to play, I just won't sprint.' I know it sounds comical."

United are aware of the allegations but have declined to comment. Ferguson will be asked for his response when he holds his weekly press conference at the Carrington training ground on Friday.

Hargreaves made his City debut in their 2-0 defeat of Birmingham City in the Carling Cup on Wednesday, scoring the opening goal. The 30-year-old believes he can play 40 times this season but feels the deterioration in his knees can be directly linked to the course of injections. "I came back a week early for pre-season to get it done. They said I would be fit for the start of pre-season but, after that, I never got back on my feet. My tendon was never the same. They said my tendon was good, but it felt like I was made out of glass."

Asked if he felt United had mishandled the situation, he replied: "You'd have to ask them. There were a few crucial points. We treated it and it got significantly worse through the injections. That obviously had a huge impact. Basically, I was left to pick up the pieces, which was incredibly frustrating.

"They said it [the injections] would help and that I wouldn't have any side effects. That obviously wasn't the case and if I'd known I could have had a reaction like that I wouldn't have done it. It was a shit position to be in, to be honest. I'm not a guy who wants to bitch and complain. I was trying to make the most of it, but it deteriorated over the season and we had to do these injections. My knee, my tendon, was never the same."

United eventually decided not to renew his contract when it expired at the end of last season and, after 39 appearances in four years at the club, Hargreaves moved to City on a free transfer in August.

The midfielder said "a change of scenery, sadly enough, was essential" for his rehabilitation. He added: "The guys here [City's medical staff] have been great so far. I've been here three weeks and you can already see the difference. I knew it was all there. It was just a matter of doing things right."