The 29-year-old will definitely miss Saturday's trip to Sunderland and Clarke admits he has no idea when Foster will be available again.
"Ben won't be fit this week. We are thinking he's got a similar problem on the other side of the groin now.
"We will get that checked out but he definitely won't be fit for Saturday."
Asked when Foster was likely to return, Clarke said: "I don't know. I'll leave that to my medical staff and we will assess it again after the weekend."