Yacob arrived at the Hawthorns on a free transfer from Racing Club in the summer, and has produced some eye-catching displays at the heart of midfield already this season.
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And, after the 25-year-old missed Sunday's 2-1 defeat to Newcastle with injury, Foster highlighted the midfielder's absence as one of the few blows the Baggies cannot overcome.
"We've got competition all over the pitch," he told Sky Sports.
"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."