That's a fair point, but I don't think it's necessarily his back. He finished second at Darlington in his first full race after he got hurt.
As @
FenderBumper pointed out, I think that Denny hasn't been the same since he lost that title in 2010. Also, perhaps even more importantly, JGR as a team hasn't really been up to snuff this year. Denny has no shortage of talent, and like plenty of other drivers in this series, give him a half-decent car and he'll usually give you a good finish. Give him a good car and he's as much of a threat to win as anyone else.
He's a headcase at times, but he won't stay down forever. Anyone that hates him like I used to better enjoy it while they can.