Manchester United's injury problems in defence have been worsened with the news that John O'Shea is likely to miss the remainder of the season because of a blood clot in a leg. O'Shea has not played since damaging a thigh in the second leg of the Republic of Ireland's World Cup qualifying play-off against France in Paris on 18 November.

The injury has since become so serious that doctors believe he could have been at risk of needing an amputation if it had gone undetected and Sir Alex Ferguson will now have to plan the next four months without a player whose versatility could have been invaluable to United at a time when injuries in defence have contributed to the team's stuttering form.

"He's got a terrible injury at the moment," Ferguson said. "It's one of those sorts of injuries that are unusual. It's like a dead leg, except a dead leg is ­usually only three or four days. The problem is that the blood clot became all knotted and when it gets knotted you get calcification so therefore it's a long process to clear that up and he'll probably miss the season now. It's a bad blow for the boy and for the team because he can play anywhere."

United have had so many defensive injuries this season there have been times when Darren Fletcher and Michael Carrick, who are midfielders by trade, have been asked to move into the back four. The Serbia international Nemanja Vidic has been troubled by a succession of calf and ankle injuries and the England defender Rio Ferdinand has not played since ­October because of a long-standing back problem and there are still doubts about when he will be in contention to return to the team.

"When you look at all the problems I've had with the back four this season, he [O'Shea] could have played every single one," Ferguson added.

O'Shea will also miss Ireland's friendly match with Brazil at Arsenal's Emirates Stadium on 2 March but Ferguson has questioned the wisdom of arranging the game in the first place. The United manager has long been frustrated about the scheduling of international friendlies at such a critical stage of the season.

"I don't even think the international managers think they are all that important now," he said. "They never play their full team and there are always drop-outs."

Ferguson, meanwhile, has spoken of the challenges of modern management and the need to rule with an iron fist. "If I lose control of these multi-millionaires in the Manchester United dressing room then I'm dead. So I never lose control. If anyone steps out of my control, that's them dead."

He added in a speech at a function at Dublin's Trinity College: "Through my development, I've come across two issues – power and control. Control is important, very, very important. My control is the most important thing."