The former England international has already had one operation, which ruled him out of the wins over Wigan, Chelsea and Sunderland.
But Foster is still troubled by the problem and looks set to miss the upcoming games against Swansea and Stoke.
"It is possible the Ben could need further surgery," said Clarke.
"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."
Clarke also has injury concerns over Jonas Olsson, who suffered a calf injury during the 4-2 win at Sunderland.
"He will go out and train later today and we will see how he goes. He is the sort of guy who will play through the pain," said Clarke.
"Jonas is one of the few players who has played every game this season but I want to ensure he is available long term."
Clarke also revealed he received a text message of support from Jose Mourinho ahead of Saturday's win at the Stadium Of Light.
The Scot, who was number two to Mourinho at Chelsea, said: "Jose said some good things in the text although what they were will remain in private."
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