David Moyes says it is a huge blow to lose Kurt Zouma on the eve of their Europa League quarter-final, second leg, against Lyon.
West Ham fear the defender could be out for the rest of the season after an ankle injury sustained at Brentford on Sunday.
It means Moyes will have to go into Thursday’s tie, which is poised at 1-1, with a second choice central partnership of Craig Dawson and Issa Diop.
Zouma has not travelled and is undergoing further tests to ascertain the extent of the damage.
"We’re still getting him checked fully and assessing it," said Moyes.
"He’s not with us here. It’s a really big blow when you lose somebody who’s a French international and World Cup winner. But it happens in football. You get injuries and unfortunately Kurt’s got an ankle sprain."
West Ham will also be without Aaron Cresswell following his controversial red card in the fist leg at the London Stadium, in which Tanguy Ndombele cancelled out Jarrod Bowen’s second half goal.
Despite that disadvantage, Moyes wants his side to draw on scalps against Sevilla, Liverpool and Chelsea earlier this season and rise to the task once more.
"We’ve shown our best quality at times in this tournament," said Moyes.
"Sevilla especially, home and away.
"It’s undoubtedly a big game, a really tough game. We’ve been good when we’ve been challenged against big teams.
"In the main, we’ve rose to the challenge."
