John O'Shea is set to miss the rest of the season, Manchester United boss Sir Alex Ferguson has confirmed.
O'Shea has not played since Republic of Ireland's World Cup play-off defeat to France in mid-November.
When O'Shea first returned to Manchester, it was believed he had not suffered anything worse than a dead leg.
But complications have arisen and it now transpires he has a blood clot and will now miss the rest of the campaign.
"He's got a terrible injury at the moment," Ferguson said in the Irish Independent.
"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.
"When you look at all the problems I've had with the back four this season, he could have played every single one."