West Ham boss Slaven Bilic has concerns over an injury Diafra Sakho picked up against West Brom.
The striker immediately signalled he was unable to carry on after having a shot blocked in Sunday's 1-1 draw with the Baggies.
"That worried me more than the result. I don't know if it's too early before the scan but it looks like a muscle injury," said Bilic.
"It's a big blow for us after Enner Valencia and Dimitri Payet."
Attacking midfielder Payet was ruled out for three months with an ankle injury in early November.
Meanwhile, Valencia has been out since July after suffering an ankle injury in a Europa League qualifying match.