It is thought that the problem is a re-occurance of the medial knee ligament problem that he suffered from during the pre-season period.
"Lee sustained some medial ligament damage and it looks like it’s a recurrence of the same injury he had early in the season." Said David Moyes
The incident opens the way for Tobias Linderoth (who should be playing before Carsley anyway IMHO) to claim a place in the starting lineup.