I've seen a few reports on twitter indicating that Adams will start but have not seen why. I'm guessing that Gilbert's injury is more severe than anticipated.
With some advanced notice, I hope that Haley can put in some plays to give Adams some extra help. If Gilbert and Pouncey are both out, that would leave Beachum as the only active backup. The Steelers will almost have to either bring up somebody from the PS or sign a veteran.