The thing about Cassell, was that he had a pretty strong supporting cast, and (almost) all the stars aligned just right that his team pulled off that 11-5 season. Watching that season though, you could tell that they VERY slowly opened up the playbook for him over the course of the season. Early on, what was available to him was enough because, in a typical season, Defenses tend to lag behind Offenses by a few games in terms of getting back to full on NFL speed. By the time Defenses started to come up to speed, they were reviewing film of him with a smaller playbook - so as that playbook opened up, it gave the team new tools that opposing Defenses hadn't quite prepared for... And that was basically how that whole season went. The only star that didn't fall into line was the lousy Dolphins taking the Division and the Pats losing some odd-ass tie breaker to not make the post-season with that 11-5 record. Who knows what they would have done had they made the playoffs, but I doubt it would've been a deep run.
And for Garopolo - kid had almost 6 quarters at the very start of the season. Nobody expected him to do much of anything, and he exceeded those expectations - again, while Defenses were in the early-season catch-up phase. Maybe he continues to develop, maybe not. Way too small a sample size to evaluate. Look at Brisette - a little more than 2.5 games, mostly still in the Defense catch-up period, and nobody would say he lit the league on fire. He almost let the Miami game get away from the team, and got shut out (albeit, with a throwing-hand thumb held on by duct tape and positive thoughts) by Buffalo. Even in the shut-out against Houston that he won - he wasn't exactly a driving force in that win.