Being physically fragile isn't a character flaw. Nor is doing what's in your own best interest as long as you do it in the right way, and that includes shutting it down for the season in my opinion.
But agreeing to be the emergency QB, dressing out, and then refusing to go in when your team needs you, most likely preventing them from going to a bowl -- I don't know what his thought process was, but that was a really bad thing to do and I'm not surprised lots of fans will not be rooting for him to succeed.
It's fair to chalk it up to immaturity, and lots of people are immature when they're young, but that doesn't change what happened or absolve him of the consequences (in this case fans of his former team not liking him.)