I believe the AFC East had the easiest schedule the year Cassell was the starter. Thats how the Phins won the Division.10-5 w/ Cassell as the starter a year afetr going 16-0 against an easier sched while missing the playoffs. Check his career record w/ and w/o Brady.
His career includes the Browns who were abysmal. Additionally, not all coaches hit on the first go round.