Jarrod Bowen faces up to six weeks on the sidelines with a broken foot suffered against Liverpool, Sky Sports News he understands.
The 28-year-old limped off in the 60th minute as Julen Lopetegui's side were hit at the London Stadium and no time frame was given for his return.
Bowen revealed he picked up the injury during the first half but tried to continue.
There were fears Bowen, who has scored six goals this season, could be sidelined for two months but Hammers boss Lopetegui is hopeful the 28-year-old will be back in action much sooner.
“Unfortunately for us, yes, he has an injury,” Lopetegui said. “He was able to play 30 minutes with this injury and now he has to rest and he has to recover.
“Let's see, but it won't be for a few days. It's not good news for us, but it's a moment for the other players we have.
“We are sure that Jarrod will recover faster because of his character, but the truth is that he will be out in the next matches.
“Let's see. I would rather not say about one number because we have to see how it develops in the coming weeks.
Bowen receives treatment on left foot during West Ham's defeat by Liverpool
“Jarrod is one of our main players, for sure, and our captain as well. But now we have to find a solution and it's time for the other players to show that they are ready to help us and to make sure that their response is going to be good.
“So we are looking for Jarrod's recovery as soon as possible, but at the same time, believing in the players who are in the squad and will try to take this opportunity.”
Bowen is West Ham's top scorer this season with five goals in the Premier League and has the most assists with four.
His injury is a significant blow for the under-pressure Lopetegui, who has struggled since taking over from David Moyes this summer.
West Ham are 13th in the Premier League with six wins in 19 games and are eight points above the top three.
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Highlights of the Premier League match between West Ham and Liverpool
Tomas Soucek is set to take the captain's armband for Saturday's trip to Manchester City.
There was some positive news from the Hammers camp this week as Michael Antonio was released from hospital following a car crash early last month.
The 34-year-old forward underwent surgery on a broken leg and will need a long rehabilitation period due to his injuries.
“Michail will definitely be out for a long time,” Lopetegui said. “But the good news is that he's home.
“He's enjoying his family and will certainly try to recover as soon as possible, but we won't have Antonio for a long time.”
What games could Bowen miss? Next matches of West Ham
January 4: Man City (a)
January 10: Aston Villa (a) – FA Cup
January 14: Fulham (h)
January 18: Crystal Palace (h)
January 26: Aston Villa (a) – Live Sky Sports
February 3: Chelsea (a) – Live Sky Sports
February 15: Brentford (h)