The experienced goalkeeper was unable to feature against Leicester City at the weekend in a match that saw the Hornets lose 4-2 which saw Daniel Bachmann start instead.
"I do not know how long, but Bachmann made some very good saves and I am calm with Bachmann in goal," said Ranieri after the game.
"Of course I hope Ben can go back early in the training sessions."
Foster has featured as Ranieri's number one since he took over the club, starting every single game, but Bachmann is now set to play until he is able to return.
Watford will be back in action on Wednesday when they face current league leaders Chelsea at Vicarage Road. body check tags ::