Their Defense is old as dirt. svttech76 thinks the same thing. No sure thing Brady will be the same either. Even if he is they won't win another SB. Don't get me wrong, they will be competitive.
Brady has a 95% chance to be the same old Brady. His injury wasn't nearly as bad as Carson Palmer's, if that's what you're comparing it to. He should be just fine after a year.
Their defense may be older, but they have a LOT of young guys in there getting experience this year. Meriweather, Wheatley, Wilhite, Mayo, Guyton, Crable, Ruud, etc. The front 3 aren't that old. Bruschi and Harrison are old, Vrabel is getting up there.. and Adalius Thomas is about as old as the front 3. Hobbs and Sanders aren't old. They've only got a few old guys, but the young guys are coming along and should be replacing the others soon. I don't have any problems about the age of that defense, it's not like they're all 11 and 12 year players. Bruschi and Harrison probably won't be around after this year, but other guys will step up and fill their shoes. The young guys greatly outnumber the older ones. So I don't think it will be a problem in years to come. Belichick is no idiot, he knows how to replace people and keep his defense up.
I think they'll be back to competing in the AFC Championship next year, but no later than the following year, depending on how quickly Brady comes along.
I still think Brady can win 1 or 2 more, it shouldn't be that hard to grasp. They might not be the team they were last year, but they'll be back on top when he's back.