Foster missed several games earlier this season with a niggling groin injury but is currently in fine form having taken his time to recover.
"We think, touch wood, that Ben is in a good place physically now," said Clarke, who succeeded England coach Roy Hodgson at the Hawthorns last summer.
"He's done a lot of work with the fitness and conditioning staff. He obviously feels in a really good place fitness-wise and I think that's important.
"I think if he'd felt there were any fitness issues, he wouldn't have come back. So the fact he's come back tells you he's in a good place physically.
"I think the fact he had two procedures earlier in the season - which went on a little bit longer than they should have done and it too him a bit longer to recover - it actually seems to have freed him up a little bit and made him more comfortable in himself. That's important, and I think you can see that in his recent performances."