Maybe a little premature but....
with the roster not set this question might need revisited in 2 months; besides BR who on this Steelers team can they least afford to lose to injury??
I am going to go with Cortez Allen. While the Steelers have picked up some young, fast interesting talent at CB, if Allen goes down this defense will feel the effect. Other than Ike Taylor, there is no proven talent of a guy that can cover the other team's #1. And as big a fan of Ike that I am, I am not sure he will be able to do it all season long if he had to.