Often time you do hear that injuries like Brady's take longer than a year to heal. This happened with McNabb in Philadelphia. However, unlike McNabb, Brady's mobility was never part of his greatness. As long as his leg is stable and he can drop back effectively, I don't see why he can't be very effective at the beginning of next year and even more effective as the year goes on.
As for Cassel, I'm very skeptical. I think some people don't understand how rare it is for a Bledsoe/Brady scenario to happen. Yes, Brady emerged out of the shadows of a (borderline) Pro Bowl quarterback to become one of the greatest quarterbacks of all time, but that stuff doesn't happen everyday. The chances of Cassel becoming 1/2 of what Brady ever was are very small. I've lived in Philadelphia the past 15 years and I've seen Eagles fans get swept up in the backup quarterback flavor of the week every time McNabb went down. Every year fans thought guys like Ty Detmer, Koy Detmer, and A.J. Feeley were going to be great quarterbacks. However, after decent seasons where the opposition didn't take them seriously, these guys started to get figured out and stopped. Just ask the Dolphins how Feeley worked out for them after they signed him away from the Eagles.
Cassel was good, but let's not forget that a lot of that is a product of the system and a product of having sensational wide receivers. Remember that Brady's numbers went up enormously last year when he had Moss, Welker, etc. to throw to. Let's also remember, that with the same receiving core Brady threw for 29 more TDs, 1200 more yards, 13 less interceptions, and a nearly a 30 point higher passer rating.
Cassel may be good, but he's most likely not going to be special. The best thing the Pats can do is try to convince some other team he is and get something for him.