Manchester United boss Erik ten Hag admits Lisandro Martinez is a major concern.
Lisandro was forced off after clutching his knee during their 3-0 defeat of West Ham.
"It doesn't look good," said Ten Hag.
"There is a bad concern. We have to wait for a few days for the diagnosis. He is very sad, very disappointed. We all feel really with him.
"First of all it is a personal disaster when it is really bad but also for the team it is really bad because he definitely brings us a lot."
Lisandro was playing in only his third league game after more than three months out with a foot injury.