Anthony Martial could miss Manchester United's next two matches after appearing to be concussed in a clash of heads at Watford.
Martial suffered the knock after 27 minutes of the 3-1 Vicarage Road defeat in a collision with Daryl Janmaat.
After treatment the France striker played on but was then involved in the incident that led to Watford's opener seven minutes later.
Martial was fouled in the lead up to Etienne Capoue's goal but no free kick was given. He remained on the floor and was then escorted off holding his head and looking groggy.
If it was concussion he would have to have a regulatory 10-day break missing the League Cup tie at Northampton on Wednesday and Saturday's visit to Old Trafford of champions Leicester City.
Manager Jose Mourinho, however, couldn't shed light on the prognosis immediately after the match.
“I don't know. I promise I don't know. You are asking me things I cannot tell you,” said the United boss.
“I didn't know why he came off whether he was injured in his knee, his ankle or his head. I don't know. I just know in that moment he is not in a condition to carry on. I don't know why I just know I had to change. I don't know yet what happened.
"I had to be on the game at that time. And not worrying with Anthony. I have not had time to see about the situation.”