Andriy Yarmolenko could miss the rest of the season after sustaining a serious Achilles injury during West Ham's 1-0 defeat to Tottenham on Sunday.
Yarmolenko was stretchered off and manager Manuel Pellegrini confirmed after the match that the winger had sustained an ankle problem, though he was unsure on the severity of the issue.
Goal understands tests have now confirmed the club's worst fears, with the 28-year-old ruled out for a minimum of six months.
It is the second long-term injury West Ham have sustained this season, with Carlos Sanchez already ruled out for the rest of the season after suffering a knee injury during the 8-0 Carabao Cup thumping of Macclesfield Town.
Manuel Lanzini also sustained a serious injury over the summer, ruling him out of Argentina's World Cup squad. He has since said that he hopes to return to action in 2019, and recently posted an update on Instagram showing him kicking a ball.
Winson Reid is also out until next year, after aggravating a serious knee injury in training.
Andy Carroll, however, is nearing a return - the club hope that he will be able to play in November - while Jack Wilshere is also on the treatment table until next month.
It adds up to a serious issue for Pellegrini, who has seen his side struggle for form over the past few weeks.
They currently lie 15th, with seven points from nine games.
The Hammers are next in action against Leicester City next week before they play Spurs again in the Carabao Cup.