I think in that 2nd quarter he absolutely without a doubt proved why he can't be depended on to be a starting QB. After he ran to the sidelines, I'm pretty certain I read his lips when he said "I just needed a break..." then when he was denied when he tried to go back in, he then said "this is bullshit..."
QBs simply don't come out of games and go back in like that. They're almost expected to tough through anything and when they do come out, they usually aren't able to go back in. What kind of starting QB comes out for a break then decides he wants back in when he technically never even won the starting job?
Won't be surprised if he "sleeps in" this week....