I ask myself if I were a senior or jr class d lineman with potential NFL skills, had had a history with knee injury, would I protect myself and my future by not giving all I had to the college that gave me the opportunity to begin with? Not to mention your brothers in the trenches all around you wearing orange? It really comes down to personal values, do I value team goals or do I value me goals? On one hand as a fan you'd like players to sell out give all they got for team wins...then again can you blame a guy for protecting his potential involving a ton of money?
If you've played the game or been around people who have played the game at a level beyond prep school, you know and/or heard playing carefully or trying to play to keep from getting hurt pretty much guarantees A. You suck in your personal performance and B. you'll catch another injury. It may be the most challenging aspect of football, coming back from an injury and finding a way to play the game with the enthusiasm it requires. I've seen a lot of talented people after a serious injury that just didn't want any more afterwards and were just there to wear the jersey.