Manchester United boss Erik ten Hag admits they need to wait for tests on Casemiro after he was forced off in defeat to Fulham.
A clash of heads with Fulham midfielder Harrison Reed saw Casimiro subsituted.
"I don't know if it's concussion but it's definitely head to head," Ten Hag said during his post-match interviews at Old Trafford.
"I actually don't know [the issue] but he had to come off. He had to come off so we have to speak with the doctor, and the medical team, to see how he's doing.
"He couldn't continue the game. We have all seen that. We had to take him off and, hopefully, he's better by tomorrow."