According to Rotoworld:

Steelers LG Ramon Foster suffered a high-ankle sprain in Sunday's win, and will be sidelined a "couple weeks."

The Steelers' offensive line continues to be stunningly injury prone. Foster dealt with a pectoral strain earlier this season, but Sunday will be the first start he's missed. Guy Whimper filled in for Foster against the Bills, and will likely get the start against the Lions. Foster has graded out as Pro Football Focus' No. 8 left guard this season, so it's not good news for Le'Veon Bell.
Quite surprised to see that Foster graded out that highly. I always thought he was at least average, but that puts him among the top 25% of players at his position. Not too shabby.