Bentley was trying to rebuild his career on loan at West Ham, for whom he made five appearances since joining on transfer deadline day.
But the 27-year-old suffered the injury last week and has now gone back to White Hart Lane.
"It's a great shame for David as he had settled well in the squad," Hammers manager Sam Allardyce told the club's official website.
"We wish him well and hope he has a speedy recovery."
Allardyce has recalled youngster Cristian Montano from his loan at Notts County as cover.
Tottenham said in a short statement: "David Bentley has undergone surgery to his left knee.
"The right-sided midfielder, who joined West Ham United on loan in August, is likely to be out for up to six months."