When a man is beating you, it doesn't matter what his rating is. Physically, Manning may be done throwing haymakers, but he can still identify defenses and beat the hell out of a lot of teams with dink and dunk football. Quarterbacks don't have to carry a team on their back, Manning is a good example of only needing to complement other talent and plays in a game.
Colleges want football to be exciting, flinging it, football. College players on opposing defenses change every year, in the pros, defensive players stay and hone their skill for years and their defensive schemes rely on disguising the defense. A quarterback that had to recognize and adjust to different defensive schemes in college is easier and quicker to learn the pro way of recognizing defenses.
The 'system' quarterbacks from college never had the experience of adjusting their plays to different defenses so it takes a lot longer learning the defenses and calling adjustments at the line in the NFL.
Rogers had years behind Favre to learn, Sanchez and Geno had none. Kaepernick had none and is hitting his wall.
I think Petty will need more than a year.
Most colleges are turning out rocket arm dumbbell quarterbacks. Colleges can't turn out even 5 quarterbacks a year that are ready to play NFL football within a year.