Michael Owen could be unavailable for up to three weeks after aggravating a groin injury in the win over Wolfsburg.
The striker sat out training on Tuesday as a precaution, but had to be replaced by Dimitar Berbatov after only 20 minutes of the Reds' Champions League clash against the German champions at Old Trafford following a recurrence of the complaint.
"Michael felt his groin on Sunday, but he trained a bit on Monday and then we gave him a rest on Tuesday to make sure he'd be available for the Wolfsburg game because it was always my plan to play him," Sir Alex told MUTV afterwards.
"Unfortunately he turned quickly on the wet surface and felt the injury.
"It's difficult to say how long he'll be out for - with groins it's usually around 2-3 weeks. But his fitness is very good and that will help."
The former Liverpool striker will no doubt be desperate to get fit for the Reds' Barclays Premier League trip to Anfield which takes place in just over three weeks' time.