You don't know that. Look at what Cassel was to what he became. Jay Cutler's arm is a lot stronger than Cassel's and he has actually had success in the NFL while Cassel was nothing but a backup. You actually think McDaniels looked at Matt Cassel before he came in as the starter and thought to himself, I can make that quarterback who btw has started a game since high school, into a good NFL quarterback. If he can make Matt Cassel into a quarterback worthy of being a starter then he can make Cutler into a better NFL quarterback than what he already was. It's also not like they lost anybody important. They had all the same WR's with the addition of Gaffney, still a good offensive line and a couple new RB's.
Worst comes to worst and Cutler can't play in the same offense that Brady was in they could've done what they did with Cassel and run shotgun 60% of the time.