Manchester United have rejected claims by Owen Hargreaves that he was given the wrong treatment for a chronic knee injury and used as a "guinea pig" by the club.

After Sir Alex Ferguson gave his backing to the medical team at Old Trafford, United issued a statement expressing disappointment with Hargreaves's comments and suggesting that the midfielder, now playing for Manchester City, was given the "best possible care".

The statement read: "Manchester United is disappointed with Owen's remarks. The club gave him the best possible care for three years and is as disappointed as anyone he was not able to play a part in the team's success at that time. It has shared all the medical records with Manchester City and is comfortable with the actions taken by its medical staff at each step of his many attempts at rehabilitation.

"United does not acknowledge any validity in the comments Owen is alleged to have made. Manchester United has some of the best sports medical staff in world sport, who have made a significant contribution to United's on-pitch success in recent seasons.

Ferguson had already backed United's "brilliant" medical staff after Hargreaves, speaking after his goalscoring debut for City on Wednesday night, claimed he felt like a "guinea pig", as he was given several injections he believes hindered his recovery during four injury-disrupted seasons at United.

The United manager responded on Friday morning, saying that although he did not know the specifics of Hargreaves's treatment, he had complete faith in the club's doctors.

"We're trying to analyse 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% 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."

Hargreaves suffered from persistent tendinitis while at United, resulting in the player undergoing double knee surgery during the 2008-09 season. Following the operations, Hargreaves played six minutes of first-team football and he was eventually released by United at the end of the last campaign.

The 30-year-old claimed the medical treatment he received at United did more harm than good. "I came back a week early for pre-season to get [the injections] done," he said. "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.

"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. That was surgery, and that was 18 months gone.

"I've had to be a guinea pig for a lot of these treatments. 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.' With hindsight it's a lot easier. And, yes, the injections I had I probably should not have had."

Hargreaves is now hoping his injury nightmare is behind him, although he is aware he will have to manage himself well in order to avoid more knee problems in the future.

"I had big surgery, so we're just going to have to be smart," he said. "My physical function is very good, it's tip-top, but sadly, I'm not 18 any more, so I need more recovery and more time in the gym to prepare for each training session.

"It's just about getting some repetition in my training and trying to find some balance, staying sharp and fit and looking after some of the issues I've had.

"I'm sad for myself that I wasn't able to play a bigger part at Old Trafford – I really envisaged it going differently. It didn't really materialise the way I anticipated, but this is a new chapter in my life."

The City manager, Roberto Mancini, believes Hargreaves could be a regular member of his squad within a month if his present progress continues. "I am happy for Owen," said Mancini. "He did very well. He played without a problem. As a first game, it was good. I hope he can continue to play without a problem. This is the most important thing.

"The crowd gave him a great reception. This guy deserves everything. After two years with so many injuries, for an important player like him to come back is really difficult. He could be an important player. If he doesn't have a problem in the next weeks, he could be part of my first team in a month or two. It depends."