The pats lost the best player in the league (arguably) and that hurts, but don't count them out just yet. From here their schedule looks pretty easy, but teams fluctuate so much year to year it's hard to know how good these teams may be down the road.
Cassel is an unknown at this point. He hasn't started a game since HS, however he's backed up Palmer/Leinart in college (ironically he beat out Leinart for the starting job, and then lost it to injury) and Brady in the NFL.....so it's not like he's been 2nd fiddle to scrubs. BB has had ample opportunity to replace Cassel and has stuck with him, so he see's something there. Also remember, we are talking about the coach that seems to plan for every possible contingency.....so it's not like he hasn't thought about this happening.
I think winning the division is very possible, and then anything can happen in the playoffs....you just have to get hot at the right time. The pats have gone from the Superbowl favorite to a very good team with the loss of TB....there are certainly teams better in the AFC right now, but it's long season and anything can happen. Realistically I can see them getting a wildcard if Cassel doesn't play great to going deep in the playoffs if he does.