He has some good tools to work with, but he needs to up that completion percentage and show that he can consistently make proper reads. It seemed like in 2015, he struggled with reads and instead chose to just throw up 50/50 balls to Mathews, Kurtz, and Houk. That worked out pretty well because all of those guys were big and tall enough to win most of the battles and come down with the catch, but you can't rely on that in the NFL.
This year will give us a better idea of how talented Tanner really is. If he can improve his completion percentage from 2015 and show he can consistently make good reads while throwing to smaller outside receivers, then I'll agree that he should be a high draft pick.