The Ireland midfielder, who has been playing in central defence this season for United due to absentees, is believed to have suffered the injury during their season-opening 1-0 defeat by Chelsea [ Images ] at Stamford Bridge [ Images ].

United have been battling with injuries, suspension and Olympic [ Images ] duties this month which have ruled out up to nine players from Alex Ferguson's [ Images ] squad.

"He's been playing with a bit of pain and it was just too much after Saturday," said Ferguson after his weakened team beat Dinamo Bucharest 3-0 to reach the Champions League [ Images ] group phase at Old Trafford on Wednesday.

"He was really feeling it. He is going to be out for three weeks which is a bit of a blow."