West Ham's memorable win over Tottenham was overshadowed by more injury worries for boss Slaven Bilic.
"It's his hamstring and it's not a couple of days (out)," said Bilic.
"We will wait until it settles down and assess it. The first information is that we should be happy if he only misses three games.
"After three games there is an international break, Easter and all that. But it's a big blow for us, the whole season he has been magnificent."