Declan Rice could be out of action for a month after sustaining a knee injury while away on international duty with England.
Prior to West Ham's 3-2 win over Wolves on Monday evening, which sent David Moyes' side into the top four in the Premier League, Rice had featured in all of the Hammers' top-flight games this season, playing a key role in their bid for Champions League qualification.
Before the game at Molineux, Rice tweeted: "Rehab has started. Gutted to be missing the game tonight. Overwhelmed by all the support but I'll be back in no time. Come on the boys!"
"It is disappointing," said West Ham manager David Moyes ahead of their triumph over Wolves.
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"We knew after the England game that he wasn't going to be available. But we move on, we've [got] over the disappointment and that feeling.
"It gives other people opportunities. That's what squads are for and hopefully we've got the squad to deal with it.
"We've moved on from it now. We know we are missing a good player but the team has been playing really well this year."
England manager Gareth Southgate will be hoping Rice makes a quick and full recovery, with the 22-year-old integral to his midfield plans for the Euro 2020 tournament this summer.
Rice played every minute of the games against Albania and Poland last week, as England made it three wins from three matches in their World Cup 2022 qualification group.
The national side's next match is a friendly against Austria on June 2, a warm-up match for their opening Euro 2020 fixture against Croatia on June 13.