"The truth is an elusive thing."
When I posted this, I was (and am) thinking the following:
JN - Beat up and barely able to walk but continuing to play with a limp and then of course breaking his arm. He was pulled for several series in the UGAg game after two UGAg players pulled on his legs in the pile.
Vad - 2013 getting beatup so bad in games that he could barely walk at the end. Taking the hit in the UNC game (as I recall) that took him out of the game for a while and, more importantly, seemed to forever make him a more tentative QB for us. I agree that Vad may have started every game, but others played a lot of snaps since Vad just got wore out. Here are the games that JT played in 2013, and you are right that it wasn't a lot.
http://www.cfbstats.com/2013/player/255/1047228/rushing/gamelog.html
TW - He was dinged up a lot and Vad was seen as the future. Here's how the games and passing for Vad in the 2012 season:
http://www.cfbstats.com/2012/player/255/1039471/passing/gamelog.html
JT - as you mentioned, he had the bad ankle which coincided with him Byerly going in. CPJ is the one who could say why for sure and I doubt he'll talk. JT wasn't full speed for the last half the season until he had the month "off" before the Orange Bowl.
So I expect JT to either get hurt so he can't start or dinged up to the extent he has to be pulled. It's just part of the game to get dinged up and sometimes more, especially when you run the ball into the teeth of the D. (Passing QBs have their own injury problems when they get sacked.) The point at which he would be pulled will depend on how effective the back-up QB is (IMHO). With Byerly out for an indeterminate time, I think JT stays in longer than he would with Byerly as the backup.
So to me, losing Byerly is a big deal becasue he would have (or will be called upon) this year most likely.