West Ham have confirmed that Kieron Dyer suffered a double fracture to his lower right leg during the Carling Cup victory over Bristol Rovers.
The England international sustained the injury following a challenge by Rovers defender Joe Jacobson during the opening stages of the Memorial Stadium clash.
The Hammers are refusing to put a timetable on Dyer's recovery after X-rays confirmed the severity of the injury but he is certain to require several months on the sidelines.
A West Ham statement on their official website read: "Kieron was stretchered off the pitch and rushed straight to hospital where it was confirmed he had fractured both the tibia and fibula of his right leg.
"It is too early to say at this stage how long the player will be out for as this will become clearer once the first stage of surgery is completed."
Dyer only joined the Hammers from Newcastle earlier this month and the midfielder's stay at St James' Park was also blighted by a series of injuries.
Upton Park boss Alan Curbishley admitted the loss of Dyer is yet another blow, but is backing his squad to cope despite a number of key players being on the sidelines.
He said: "The game was irrelevant after Kieron's injury. I'm really down because he was looking sharp out there, playing in his favourite position in the middle of the park.
"Everyone in the dressing room's really flat but we've just got to get over this. After all, that's the reason why we've got a squad and now we've got to push on and look forward to Saturday's game at Reading.
"Although you sometimes wonder why you put such a strong team out for these games, it still won't make me think twice about putting out a similar strength side in the future."