Cape Town - West Ham expect Andriy Yarmolenko to be ruled out for approximately six months after he underwent surgery on an Achilles injury on Monday.
The off-season signing from Borussia Dortmund suffered the blow in the London derby defeat to Tottenham at the London Stadium on Saturday.
Yarmolenko was carried off on a stretcher in the 40th minute after appearing to get his studs caught in the turf and he looked in some discomfort while receiving medical treatment inside the opposition penalty area.
The 28-year-old winger, who has scored two goals in nine Premier League appearances for the Hammers, could return to action before the end of the season if all goes well with his recovery.
"West Ham United can confirm that Andriy Yarmolenko has undergone successful surgery to repair a damaged Achilles tendon," a statement from the east London club read.
"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."
Irons 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."