Foster praises vital Yacob
Argentine Yacob arrived at the Hawthorns on a free transfer from Racing Club in the off-season, and has produced some eye-catching displays at the heart of midfield already this season.
The 25-year-old missed the 2-1 defeat to Newcastle through injury on Sunday, prompting Foster to highlight the midfielder's absence.
"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 percent 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."
West Brom are back in action at home to struggling Southampton on Monday.