Manchester United manager Erik ten Hag has said he is fearing the worst after Lisandro Martínez suffered a knee injury during the 3-0 win over West Ham on Sunday.
Martínez was forced off in the second half after West Ham defender Vladimír Coufal fell onto the Argentina international's outstretched leg.
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The United centre-back initially tried to carry on but broke down a second time and was replaced with Raphaël Varane.
Ten Hag called the injury a "personal disaster" for Martínez, who was making his third start after more than four months out with a foot injury.
"I can't say [how bad it is] but it doesn't look good so there's a big concern," Ten Hag said.
"We have to wait for the coming days for the diagnosis then we can tell you more. We do everything to get the right diagnosis to see what the damage is.
"He is very sad, very disappointed. We are with him. It's a personal disaster. Let's wait but it is really bad."
The injury to Martínez was the only downside for United on a day when they recorded their biggest home league win of the campaign.
For the first time this season, Ten Hag's team won a Premier League game by more than a single-goal margin thanks to Rasmus Højlund's fourth goal in his last four games and two from Alejandro Garnacho.
"It's only one game, so don't celebrate this too much," said Ten Hag.
"We don't have reasons to do it but for today we are pleased with it. Now have a team that is in balance on the pitch. We can play better but also you see the progress is coming.
"It's not about one player, it's about a team. That's what I have seen today. It looked like a team. Eleven defending, eleven attacking. For me, it was enjoyable to watch."