Foster was forced off 12 minutes from full time with an ankle injury during the Baggies' goalless draw at Everton in the Barclays Premier League.
The 32-year-old had otherwise been in superb form, producing a number of fine saves to preserve a clean sheet in front of watching England manager Roy Hodgson.
Manager Steve Clarke said: "In his own words, he felt a pop in his ankle. He said the pain was obviously strong.
"Ben is not the type of guy to go down if there is nothing wrong.
"The fact he went off the pitch tells you it is an injury we have to be a little bit worried about.
"At the moment until it is diagnosed we will let the medical people do their job and have a look at the scans.
"It is difficult to say what the prognosis is or the outcome.
It is best to say it is just an injury that worries me."
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