Matt Cassel isn't Tom Brady, nor will he ever be.
He hasn't got the talent, the intensity, the smarts, the decision making ability, or the single minded will to win and competitiveness that Tommy boy has.
I do hope however that he will be an effective game manager, which is all the Pats need him to be. Get the run game going (Pats were averaging 4.5 ypc last year, and that was up to around 5.5 ypc against KC on Sunday), and make sure that Cassel doesn't have many third and forever's to make. Rule number 1 for him though is don't turn the ball over.
If he begins to turn it over, then we'll be in trouble, and so will his career.