"He had a little setback which pushed him back a little bit," Pulis told reporters.
"It'll be Christmas time I think. We've got to make sure we're careful," the manager added.
Boaz Myhill has come into the Baggies' squad in Foster's absence, and the team, who host Everton in the Premier League on Monday night, currently sit 12th in the table after six games.
Stoke City's Jack Butland has taken Foster's England place as back-up to Joe Hart and although the keeper will be anxious to force his way back into Roy Hodgson's squad ahead of next year's European Championships in France, Pulis has advised the 32-year-old to focus on his club career first.
"I think, having spoken to Ben, the most important thing is for him to get fit and start playing for West Brom again and if the Euros come for him then brilliant," Pulis said.
"The most important thing is he's got to get fit and get back into the team.
"He's up and running again now so fingers crossed he'll be ready to go before Christmas."
(Reporting by Simon Jennings in Bengaluru; editing by Sudipto Ganguly)
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