Matt Cassell proved the same thing Steve Young did, with the right system you can mitigate the effect your QB has on the system.
Steve Young inherited a dynasty and was a HOF QB yet only won one SB thanks in large part to the Jimmy Johnson-Jerry Jones falling out.
How did that Belichick system do w/o Brady in Cleveland or in NE for the first 18 games? People forget Cassell won 10 games last year- that's 6 less than Brady the year before. That's a HUGE difference especially since the sched was weaker and Cassell had been in that system for years backing up Brady so he knew the system.
Now that's even crazier. Manning is the smartest player out there. He constantly audibles and changes his recievers routes based on the defense. In the 2006 playoff game against the Patriots, they Pats had gameplanned for weeks on a new type of coverage to confuse and stop Manning. But by the end of the game, he found the weakness, and exploited it on the final drive to win the game.
Manning reads defensive coverage better than anyone by far. I would even say Brady is closer to Manning physically than mentally.
And he used to melt in the playoffs early in his career, not so much anymore.
He's fragile mentally.
Not so much anymore? nah, just the last 2 years after winning the SB. His 2 INTs killed Indy at home 2 years ago and last year he made one play all game when SD was asleep and had a chance to end the game w/ one first down but couldn't do it.