Ghanaian star Andre Ayew faces up to four months on the sidelines as a result of the thigh injury sustained in Monday’s Premier League match at Chelsea.
Ayew who joined the Hammers from Swansea for a reported 20.5m lasted just 35 minutes of his debut at Stamford Bridge before limping off.
The Hammers have not given a time frame for Ayew’s absence, saying last night that he is facing “a significant rehabilitation period”.
However, Jack Sullivan, the son of West Ham’s joint chairman David Sullivan, earlier wrote on Twitter: “We are devastated that Ayew will be out for about 4 months. He needs surgery.”
The news will be a major blow for manager Slaven Bilic, who had spent all summer attempting to bolster his attack before finally landing Ayew last week.
He is already without another new signing, Sofiane Feghouli, through injury for the Europa League play-off first leg against Astra Giurgiu, while Aaron Cresswell and Manuel Lanzini are also sidelined.
In addition to the Hammers’ injuries, new left-back Arthur Masuaku is cup-tied, while the influential Dimitri Payet did not travel.