Hargreaves returned to competitive football with a goal in Manchester City's 2-0 Carling Cup win over Birmingham on Wednesday.
Renaissance man: Owen Hargreaves made a succesful, goalscoring return to football against Birmingham for United's bitter rivals
Released by neighbours United at the end of last season, his career looked to be over after knee injuries and muscle problems restricted him to 26 starts in four years at Old Trafford.
But the 30-year-old midfielder claimed that a series of injections at United in the summer of 2008 ruined his time at the club.
Final outing: Hargreaves winces after his fateful appearance against Wolves - his last in a Manchester United shirt
Hargreaves said: 'I had a bit of a tendon problem. I played and it got a little bit worse. We treated it and it got significantly worse after that through the injections I had. After that my tendon was never the same.'
The injections of a solution into ligaments is an established medical procedure aimed at stiffening and strengthening the tissues, but Hargreaves added: 'After the injections, they said my tendon was good but it felt like I was made out of glass. That obviously had a huge impact.
'That was surgery and that was 18 months gone.'
Pain barrier: Hargreaves tussles with Andres Iniesta in the semi-final of the Champions League a few months before his injections
Hargreaves stressed that he holds no grudges against United's staff but it is clear that he blames the prolotherapy injections he had while at the club.
There is no evidence that Hargreaves' injury was misdiagnosed or of malpractice on behalf of United.
Promising start: Hargreaves departs the field to a handshake from Roberto Mancini after 57 minutes
Hargreaves added: 'Everybody tries their best. They said that I wouldn't have any side effects from the injections. That wasn't the case and if I'd known, I wouldn't have done it.'
Manchester United refused to comment.
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