Thats a silly way to look at it. Points per game is as much a measure of the defense as it is the offense. The Patriots offense this year had drastically worse starting field position than the team last year because the defense was so bad.
According to DVOA, the Patriots offense last year was the best ever, at 42.6% DVOA. The Colts offense has traditionally been about 30%. The best teams this year were around 23%. Now, I don't think there was any way the Patriots offense wasn't going to have some amount of decline. I'd guess they would have been 35% or so this year, slightly better than the 2006 colts. Best offense in the league by a large margin.
They finished this year at 15%, good for 9th in the league. If you use weighted, which weighs the end of the season more than the beginning, they were at 21.7, good for 5th in the league. Theres a very good chance, that with Cassell as a starter next year, they're the best offense in the league.
Yes, right now Cassel is still a drop off from 100% healthy Brady, but if Brady can't start immediately, and Cassel plays 4 preseason games and 6 more regular season starts, I'm not sure that can still be said. If Brady can't move in the pocket like he used to, then I doubt that it can.