The best way to view this situation is that as a DC w/ the Rams, they performed well. As a HC his Chargers unit didn't, and it also came w/ complaints from several players that it was because he overcomplicated things.
What's the difference between teams outside of a lockdown corner and Aaron Donald?
As a HC there was no one to reel him in. W/ the Rams he had to do what was requested of him by the HC. The same would be here w/ Kellen. If Kellen wants his defense to be fast and physical Staley would be wise not to put himself in the cross-hairs by doing anything less than Keeping things simple to meet the HC's vision.
Still don't like the things I've read about him, but he gets equal amounts of support and disdain amongs former players so the easiest thing to do is give him a clean slate here and hope for the best