The defense took a big step when Pettine took over as coordinator and took another step last season. Is that because Pettine left or is it even related? I doubt it. It speaks more to continuity, added experience (especially Dareus, Bradham, and Searcy), some important new pieces (Spikes, Brown, etc), and some good depth.
I have no doubt Pettine would have had the BILLS defense at the top of the heap if he'd stayed around another year.