The Tottenham midfielder, currently spending a season at Upton Park, is set to be sidelined for half a year after undergoing an operation on his knee
The Tottenham man had fallen out of the spotlight since making his move from Blackburn to White Hart Lane, but was hoping a season-long loan at the Hammers would breathe new life into his career.
As well as being a massive dent in Bentley’s plans, Hammers boss Sam Allardyce is now a player down, and the former Bolton boss spoke of his sympathy for the midfielder.
He told the club's official website: "It is a great shame for David as he had settled in well to the squad. We wish him well and hope he has a speedy recovery."
Since finalising the temporary move on the final day of the transfer window, the winger had made five appearances for the club, but it now looks unlikely he will be able to play a big part in the club's push for promotion.
Bentley is not the only player the Hammers have lost to a long-term injury recently, as keeper Robert Green was ruled out for six weeks forcing the club to secure an emergency loan move for Arsenal’s Manuel Almunia.