The Dutch keeper sat out the win over Aston Villa at Old Trafford on Saturday after aggravating a groin complaint.
Ferguson has admitted the injury is becoming a concern for United and is likely to rule him out of the European quarter final.
"Edwin van der Sar may be a doubt for Tuesday," Ferguson told Setanta Sports. "He's got a groin problem again, it's happened two or three times in the last few weeks.
"We'll see how he is over the next few days. We did not risk him today which was a good thing. We could have risked him and it got worse. Not risking him today gives us a better chance."