The problem is a double edged sword: "Back in the day," the running game ruled, and so QBs were not as highly valued as they are today. There was less rush to get them on the field as long as you had a solid backfield. Nowadays, the passing game rules AND is more complex. So the number of QBs who are capable of succeeding in this league is much less than years ago, and average QB play is less acceptable today. Thus, QBs are pretty much expected to hit the ground running and they get usually one year to break bad rookie habits.
Mann absolutely has to go immediately. There's got to be some journeyman out there that can do a better job than him.