Lukasz Fabianski is likely to be out for at least three months with a torn hip muscle, says West Ham boss Manuel Pellegrini.
The 34-year-old goalkeeper suffered the injury in West Ham's 2-2 Premier League draw with Bournemouth on Saturday.
"Of course it's a big blow because Lukasz was performing like, for me, probably one of the best goalkeepers of the Premier League," Hammers boss Pellegrini said.
"The medical staff will decide properly today or tomorrow if he will have surgery on his muscle or if he will recover without surgery, but I think in both cases anything less than three months will be difficult for Lucasz to return."
However, Pellegrini is confident Roberto Jimenez will be a capable replacement in Fabianski's absence, and says he will not be looking to add to his options while the Pole recovers.
"Absolutely confident because Roberto is a goalkeeper that has had a brilliant career, not only in Spain - he also played in Portugal, Greece, and for other teams in Spain. We brought him here because we know him and we trust him," said Pellegrini.
"We also brought in David Martin, another goalkeeper with experience in the league so, for the moment, we are going to stay the same."
Live Premier League
West Ham are unbeaten in the Premier League since losing to Manchester City on the opening weekend, and on Saturday host Crystal Palace, live on Sky Sports.
The Hammers are fifth in the league after seven fixtures, and while Pellegrini has urged his players to take things one game at a time, he insists it is important to believe they can maintain their position in the top five.
"I always believe," said Pellegrini. "You must have a mentality that reflects that you are at least going to attempt to do it - after that you will see if you arrive or not.
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"In the way we are playing I think that we are improving a lot, and that's one of the reasons why we believe we can do it.
"Seeing the team playing the way we did against Bournemouth - we went away and from the first minute we tried to win the game."
Watch West Ham vs Crystal Palace live on Sky Sports Premier League from 5pm on Saturday