West Ham defender Kurt Zouma was forced off during defeat at Crystal Palace today.
The Frenchman is now a doubt for the Europa Conference League semifinal against AZ Alkmaar.
Zouma will face a scan on Sunday.
The stopper suffered the injury while trying to prevent Jordan Ayew's 15th-minute equaliser and had to be replaced soon after by Angelo Ogbonna.
It looks as if he's gone over on his ankle," Hammers boss David Moyes said. His ankle's swollen up at the moment, we'll get him checked up tomorrow with a scan."