he most likely doesn't underthrow that ball. He's getting (I think) some unwarranted flack for underthrowing (one of the fastest receivers on the team), a bomb, which ultimately was completed for the biggest offensive play of the game.

IF he gets the reps in practice that QB1 or even QB2 gets...he doesn't underthrow that ball. Considering what he is called to do, with the bare minimum reps he must get in practice...that was a phenomenal play/pass.