The midfielder, a summer signing from Racing Santander, has played a key role in Albion's good start to the season under Steve Clarke.
Yacob was injured for Sunday's defeat at Newcastle, and goalkeeper Foster reckons the Argentinian should be the only man in Clarke's squad guaranteed a starting spot.
"We've got competition all over the pitch," he said. "That's fantastic for the team because you know if you're not performing at 100% there is somebody waiting to take the position off you.
"I have said there is nobody who can be guaranteed first-team football but that is probably apart from Claudio.
"I don't think you can cover Claudio because he has been absolutely immense. We did miss him at Newcastle."