Sir Alex Ferguson has accused Wayne Rooney of showing no respect for Manchester United and personally letting him down as he opened his heart to talk of his devastation about the player's desire to leave Old Trafford.

Ferguson said he found the striker's decision "inexplicable" and though he insisted United would keep a contract offer open, there was no disguising his shock and disappointment, describing himself as "dumbfounded" as he spoke at length and with great emotion about the way he felt the club had tried to protect Rooney from damaging stories about his personal life.

On a day of remarkable drama at Old Trafford it also emerged Rooney had been taken off the training pitch on a stretcher after suffering an ankle injury. The current footballer of the year left Carrington on crutches, continuing a dismal season in which his form has totally deserted him and it has become clear he is not only determined to sever his ties with United, but is also willing to follow Carlos Tevez's path to Manchester City.

"I'm disappointed, very disappointed, I can't believe it," Ferguson said. "He had told us he was happy at the club, that he was at the best club in the world, so I must say it was terribly disappointing to get the news because I can't quite understand it.

"It's a club that no one can deny is one of the most successful in British football. We have won 40 major trophies, been to countless cup finals, we have a fantastic history, a great stadium, great training arrangements. There's a platform for anyone to take up a challenge here. There is incredible romance at this club. We just don't understand it."

Rooney is out of contract in 20 months and, according to Ferguson, had asked for an extension before everything came to head on 14 August, two days before United's first game of the Premier League season, against Newcastle United, when Rooney's agent, Paul Stretford, told the club's chief executive, David Gill, that he wants out.

Ferguson said: "I was at Carrington and David phoned and said: 'I'm coming over, I have some bad news for you, his agent has intimated that he won't be signing a contract and he wants away.' I couldn't believe it either. I was dumbfounded. It was a shock."

Ferguson admitted he was particularly hurt by the suggestion from Rooney's camp that he wants to leave because of irreparable differences with the manager. "There's been no falling out. We've not had any argument, not a bit. But I think that you have to understand the mechanics of these situations, when people are working to leave a club. It's an easy one to say you've fallen out with the manager. It's a very easy one to say that.

"I had a meeting with him and he intimated it to me [that he wanted to leave] in his own way. I said: 'The only thing I want from you is to respect the club's position and its traditions and behave like a proper professional and we will try to see it through.'

"And, you know, I don't know if he has done that. I have my doubts about that because reading all these things about him falling out with me and all that nonsense then it's disappointing because we have done everything we possibly can to help Wayne Rooney.

"Since the minute he's come to the club, we've always been a harbour for him – any time he has been in trouble, the advice we have given him, we have done nothing but help him. I was even prepared to give him financial advice, many times. I don't know how many times we have helped him in terms of his private life and other matters.

"But we've got into a situation now where we have to clarify it for our fans. What we saw [against West Bromwich Albion] on Saturday was unacceptable, the minute we got to 2-2 and the fans were chanting for Wayne Rooney. That puts pressure on the players and didn't do any good to the team. So we had to clarify the situation and put it right."

Ferguson went on to accuse Rooney of being deliberately economical with the truth when he came out after England's game against Montenegro last week to say that he had not been suffering from an ankle injury this season, in direct contradiction to the manager's statements of the previous few weeks.

"It was disappointing because when you know a player's form is a little bit down, you may say he has an injury to protect him, because you don't want to humiliate him and you have respect for him. In a lot of cases over the years we have done that, but in this case when we came to the Sunderland game we decided to give him a complete break to get fit and to play for England, play at Wembley, play for his country and I was hoping that we get his form back.

"That was my intention – to try to galvanise him in terms of getting a proper break from the games, get the training done, get his fitness back, get rid of the little traces of the ankle injury and then play for England. Why he has come out and said that, I have no idea."

Ferguson was speaking at a press conference to preview tomorrow's Champions League tie against Bursaspor. He began by answering questions about the match but when the subject turned to Rooney he said he would read a statement before eventually baring his feelings, uninterrupted, without any reference to his notes.

"I can't answer the question why he is doing it. You can speculate and have opinions, but it won't matter a dickie bird simply because the player is adamant in saying that he wants to leave.

"I did say to him and David Gill has also intimated that there is an offer there if he wants to accept it. It's an offer that never got into financial discussions, ie money-wise, but David was prepared to offer him a contract that couldn't be bettered elsewhere.

"There has been no argument, but we have got to a stage now where I feel that we still have to keep the door open for him because he is such a good player."