Tua's arm strength is much better than most think. Go look at his college tape. In Miami, he has been forced to get rid of the ball in a hurry so rarely has he been able to throw downfield. On top of that, he should never played his rookie year except some mop up duty, his hip was not ready at that point. Then he played most of last year with a couple busted up ribs and a broken finger on his throwing hand. All courtesy of the right tackle letting him get creamed at the beginning of the 1st Bills game.
It comes down to this with him now, Grier has addressed just about as much as one could in the offseason and given better protection and better skill position players. If Tua does not show much better this year they WILL draft his replacement next year. That's why they have two 1st round picks in the bank next year so they can package them with others to move up and get a different QB. So it's all about Tua this year.
Bridgewater is their insurance policy. If Tua struggles, I'm certain he will be pulled quickly.