He's been legitimately hurt. He keeps getting legitimately hurt.
It's consistently been lower body injuries with him. Some guys are built to take punishment and some guys aren't. It sucks, and I don't know if he's ever going to be 100%.
I don't think the problem with Parker is his heart or his head. I think he's just what some people would call a candybone. Then again, a lot of people feared that was the case for Aaron Rodgers early in his career, and I'd say he's still been worthwhile for the Packers.