The 31-year-old, who trains on his own due to a chronic knee injury, was substituted during the 1-1 draw at St James's Park on Sunday.
"He's got an abductor strain," said Redknapp. "He's a fantastic player but his knee blows up like a balloon every time he plays so he can't train.
"His body is nowhere near fit so he's always open to pulling muscles and getting strains. That's what's happened today.
"I wouldn't like to say (how long he will be out for) but it will be a while I think."
Spurs continue their Europa League campaign on Thursday when they take on Rubin Kazan at White Hart Lane.