I look at it this way,he's been playing behind terrible protection,and bad WR's for 4 years...you know about all that...and the only time he's missed was that 6 gm's with a high ankle sprain from getting crushed in the pocket over and over again trying to learn a new offense every year. Before that the shoulder injury at OU,which hasn't re-materialized since.
IMO the ACL was just one of those things that happens. He's a pocket QB who can move around outside the pocket better than he can inside the pocket it seems. We've seen him take some vicious hits,and he gets right up. Injury prone is a term i save for guys like Saffold,who can't take a hit(physicality of the game in relation to his position).
Sam's biggest hurdle in 2014 will be to get his team to the playoffs. I don't see evidence that Sam is a fragile QB. He just needs to get better.