Um, nagging hip issues doesn't explain a defenseman who seems to forget the basic tenets of playing defense once his ice time creeps past the 10 minute mark.
Gunnarsson is the classic tweener. He's passable offensively and passable defensively, but you can make up for that using defensive pairings. Since he doesn't excel at either, he's best used as a bottom pairing defenseman simply because you minimize risk that way. If you try to use him as a top pairing defenseman, his presence tends to magnify the flaws of his defensive partner, since then the whole pairing has a major weakness in whatever his defensive partner isn't skilled at. Hence the constant *****ing about Phaneuf; it's not that Phaneuf is bad, it's that Gunnarsson isn't good enough to mask Phaneuf's weaknesses, which should be the goal of every defensive pairing.
This tends to be why most teams relegate D-men like Gunnarsson to the bottom pairing. The Leafs used to have a Gunnarsson-type D-man back in pre-lockout; Aki Berg. Why do you think Berg never progressed past being a #5D? After all, he was a high draft pick specifically because of his balanced traits.