Foster has already had one operation, which ruled him out of the last three games against Wigan, Sunderland and Swansea. But the former England international is still troubled by the problem ahead of Wednesday's away clash at Swansea.
Clarke said: "It is possible that Ben could need further surgery. If that is the case, it will be the same timescale as before, that it will be seven to 10 days."
Clarke admits his main thoughts at the moment are on current keeper Boaz Myhill rather than Foster. He said: "I have not spoken to Ben. He is with the medical staff. He does not need me to hold his hand and tell him I love him!
"He knows how important a player he is to us but I've every confidence in Boaz.
"His attitude was excellent when he was out of the side and he has grabbed his chance with both hands."