Andriy Yarmolenko will be out for six months after tearing his Achilles tendon in West Ham's 1-0 home defeat to Tottenham on Saturday.
The Hammers winger went down inside the Spurs penalty box in the 40th minute after appearing to catch his studs in the turf.
Yarmolenko was carried off on a stretcher and, after being assessed, underwent surgery in London on Monday.
A club statement read: "West Ham United can confirm that Andriy Yarmolenko has undergone successful surgery to repair a damaged Achilles tendon.
"The Ukraine international sustained the injury during Saturday's 1-0 defeat against Tottenham Hotspur at London Stadium. He had an operation on Monday morning and is expected to be sidelined for around six months."
Head of medical services, Richard Collinge, added: "Andriy's operation went well and we will now work hard to support him throughout the rehabilitation process.
"His recovery will be planned very carefully and we have every confidence that he will return to full fitness within the scheduled timeframe."
After the defeat to Spurs at the London Stadium, West Ham boss Manuel Pellegrini admitted it would be a big blow to lose Yarmolenko with fellow midfielders Jack Wilshere, Carlos Sanchez and Manuel Lanzini already on the sidelines.
"It's a pity because we have five players with long-term injuries - Lanzini, Wilshere, [Andy] Carroll, Carlos Sanchez and now we could have Yarmolenko," he said.
"Four of them are midfielders so it's not easy but we have to continue working with the players we have."
Yarmolenko joined West Ham from Borussia Dortmund in the summer for £17.5m and has since scored two goals in 10 appearances.