West Ham winger Andriy Yarmolenko is expected to be out for six months after having surgery on a torn Achilles.
The 28-year-old, a £17.5m summer signing from Borussia Dortmund, was injured in the first half of Saturday's home defeat by Tottenham.
Head of medical services Richard Collinge said West Ham had "every confidence" that Yarmolenko will return "within the scheduled timeframe".
The Ukraine international has scored two goals in 10 games this season.
After Saturday's game, West Ham manager Manuel Pellegrini said: "It was a pity because at this moment we have five injured players and four of them are midfielders, so it's not easy."
Arsenal defender Laurent Koscielny suffered a similar injury in May. The 33-year-old former France international has recently been pictured back in training and is expected to return in November.