West Ham striker Diafra Sakho requires surgery on his thigh injury and faces up to three months on the sidelines.
The Senegalese forward picked up the problem during last weekend's 1-1 draw with West Brom and had to be substituted in the closing stages of the game.
Although an early diagnosis suggested the 25-year-old would be out for just two months he visited a specialist earlier this afternoon and it has been confirmed he will require an operation.
A club statement said: "Diafra Sakho consulted a specialist on Thursday afternoon to further assess his thigh injury.
"The Senegal international will undergo surgery on Saturday.
"The expected rehabilitation time from the operation is expected to be approximately three months."
Sakho's has scored three goals in 12 top-flight appearances this term and his injury has left Slaven Bilic relatively short of attacking options ahead of Saturday's trip to face Manchester United at Old Trafford.
Both Enner Valencia and Dimitri Payet are sidelined with Andy Carroll or Nikica Jelavic expected to be given a chance to lead the Hammers' front line against Louis van Gaal's side.
If Sakho is out for the full three months he will miss a total of 14 Premier League fixtures.
