Cassel has the measurables and I don't doubt that he can be at least an average QB. Lets see what he does when he has to constantly go through his progressions.
I have a feeling that this game might show folks that having "the measurables" is over-rated. There are a lot of QB's that had the measurables that aren't in the NFL anymore. Anyone remember Jim Druckenmiller or Ryan Leaf? Tom Brady is clearly an elite QB and the quality that puts him there is his decision making and coolness under pressure. It's not impossible to rattle Brady, but it sure ain't easy. However, the Jets did make things difficult for him, in their last meeting, and I have a feeling Cassell will have everything thrown at him that the Jets can muster. I bet Belicheat is thinking some illegal videotapes would really come in handy this week.
Neither of those two had the brain power or the mental strength to handle being a backup QB for 4 years in college. Cassel's mental fortitude to persevere is more than those two combined.
What I find funny about this whole situation is that Patriots haters used to discredit Brady's personal achievements by downplaying his role and arguing that he has been the beneficiary of great defense and coaching his entire career. Now that he is hurt, all of a sudden he's the only decent player on the roster, and the Patriots can't possibly compete for a playoff spot without him.
Brady did benefit greatly from defense when he won. Individually he's always been good of course. Nobody is saying he's the only decent player, but if you don't even have a mediocre QB replacing him you're screwed. Cassel has a shot at the playoffs with Moss by his side.
There is a difference that you are glossing over. Brady is downplayed when it comes to comparing him with Peyton and other all time greats. He isn't downplayed when it comes to comparing him to an average QB. As for making the playoffs, the AFC is very tough and even the slightest misstep is going to hurt a team. 9-7 likely won't be good enough for a Wild Card spot.