The Ukraine international had to be stretchered off shortly before half-time in West Ham’s 1-0 loss to London rivals Tottenham Hotspur on Saturday.
Manager Manuel Pellegrini confirmed after the match that Yarmolenko had suffered the injury “between his foot and his ankle”, with early medical tests confirming a problem with his Achilles.
The 28-year-old, who joined West Ham from Borussia Dortmund in a £17.5m move, suffered a similar injury during his time in the Bundesliga.
He is expected to undergo surgery next week before beginning his rehabilitation.
It is the second injury blow West Ham have suffered this week, following the news that Jack Wilshere’s return to first-team football will be delayed.
Wilshere needed an injection last week after he felt soreness around the ankle on which he had recently undergone surgery.
The former Arsenal man had been hoping to return against Tottenham in the Carabao Cup next week, but is now not expected to play until November.
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