Jonny Evans suffered a recurrence of his ankle injury during Manchester United's win over Blackburn Rovers.
Evans faces tests on the injury later on Sunday to determine whether he will be fit to face Inter Milan in the Champions League on Tuesday.
Sir Alex Ferguson rates the Northern Ireland international ''very doubtful''. But he will give Evans every chance because - with Nemanja Vidic suspended, Gary Neville and Wes Brown definitely out and John O'Shea absent from United's latest win due to a heel problem picked up against Fulham - he is facing a defensive nightmare at the San Siro.
Evans missed four matches with the original injury - and since he was also set to feature in next Sunday's Carling Cup final against Tottenham, he can only hope the damage is not as bad this time.
''It is a recurrence of the ankle injury I had before and is a bit tender,'' he admitted.
''I caught it when I kicked the ground during the game.
''At this point, I don't know whether I will be fit for the San Siro. The doctor said he will have a look at it - and we will have a better picture then.''
If neither Evans nor O'Shea is fit, Ferguson would be left to choose either rookie reserve-team central defender James Chester against the Serie A leaders or play one of his more established first-team stars out of position.