I wasn't there, and I don't have any evidence. But looking at his overall body development, it doesn't look like he's done a lot of strength training in the years since the Spurs drafted him. It's easy to believe he didn't rehab as hard as Bertans. He was so young, and I kind of wonder if he had someone pushing him the way he would have if he was actually in the Spurs' organization.
I don't know if they can still salvage him if he stays in Austin. But maybe getting the contract will give him the financial stability to stay and try.