The Portugal international winger missed the entire festive programme and remains unavailable, but the news is more positive for right-back Rafael, who had to be withdrawn with a groin problem during the entertaining Boxing Day triumph at Hull City.
"Nani has got a hamstring injury which is probably going to keep him out for quite a while," revealed manager David Moyes to MUTV, in an interview that is available to watch in full on MUTV Online.
"It's a serious hamstring injury that he picked up in training.
"I don't think Rafa is too far away, though, as his injury isn't too serious. Hopefully, we're beginning to get one or two back."
Chris Smalling has been deputising for Rafael in the right-back position, although winger Antonio Valencia has also reverted to the role during games.