Marko Arnautovic will be out for around a month with a hamstring injury, West Ham manager Manuel Pellegrini has confirmed.
The Austria forward picked up the injury during Tuesday's 3-1 win against Cardiff and looks set to miss the busy Christmas period.
Pellegrini said: "Arnautovic has a muscle injury and he will probably be out for one month.
"We know how important Marco is for the team. But we have a squad and it is a moment for other players to demonstrate why they are here.
"I was very happy with Lucas Perez the other day, that he took his chance and scored two goals, so we must continue working with the players we have at this moment."
The Hammers are resurgent, having recorded back-to-back wins, but Pellegrini has a reduced squad to select from for Saturday's London derby at home to Crystal Palace.
Pellegrini said: "We have too many players injured - in a month where we know we have to play seven games and then six games more in January.
"Out for this week are (Ryan) Fredericks, (Winston) Reid, (Jack) Wilshere, (Manuel) Lanzini, (Andriy) Yarmolenko, (Carlos) Sanchez, Arnautovic.
"Wilshere's ankle is not 100 per cent comfortable, so he will not be involved in this game, but next week he may be OK."
Andy Carroll is back in contention, however, having come off the bench for 26 minutes at the end of Cardiff match, as he recovers full fitness following an ankle injury.
Pellegrini said: "Andy Carroll played some useful minutes the other day, we hope he will play some games in a row.
"He worked today without a problem."
Despite the injuries, the Hammers have hit form and, currently 13th, they will obviously be determined to pull further clear of the drop zone.
"We are doing things better but we cannot stop at this moment," said Pellegrini.
"We had such a bad start at the beginning of the season, with not one point from 12, that we must continue trying to recover."