Romelu Lukaku faces two games out, according to Manchester United boss Jose Mourinho, after he was stretchered off following a blow to the head against Southampton.
The Belgium international collided with Saints defender Wesley Hoedt in the ninth minute of the goalless draw at Old Trafford.
Lukaku was tended to by medical staff for as many as five minutes before he was carried off to a standing ovation, while receiving oxygen.
At full-time, Mourinho said: "I don't know [how he is]. I know that, normally, bad news arrives immediately, and it didn't arrive.
"But to leave the pitch in the way he did, with the doctor not thinking twice to make the decision immediately, is because there is a problem, not just for today but for the next two matches, I would say, for sure."
United face a crisis up front with Zlatan Ibrahimovic, having only just recovered from a serious injury, confirmed to be out for a month.
Both players are set to miss the crucial Premier League game against Everton on New Year's Day, as United again dropped points in the top-four race and fell behind Chelsea into third.
Lukaku is also a doubt for United's FA Cup third-round clash with Derby County while Ibrahimovic will miss at least a further two games.
Prior to the injury, the Belgium striker came close to giving United the lead on a frustrating Saturday night for Mourinho's side.
The Portuguese said: "We started the game and lost the striker, and we lose one of the only two changes on the bench (Ibrahimovic) in terms of attacking areas, leaving me with one change for the rest of the 90 minutes."