This all to me is easy. Rosier is the starter at the beginning of the season. As long as we're winning or he's playing decently, he stays the starter. Williams/Perry or both get reps in garbage time against teams like Savannah State. Rosier continues to start until he hurts us, in which case, the next man up has some game experience and is by and large ready.
Unless we're undefeated/one loss, gut feeling tells me Jarren is starting games at the end of the season. Might take some time to grasp the playbook but the consistency and accuracy from day one is far and away beyond Perry and Rosier.