In 2008, Cassell threw for 21 TDs. For comparison, in 2007 Brady threw for 50. 2007 16-0 versus a very strong schedule, 2008 11-5 versus a historically weak schedule. Cassell was on a stacked team and put up average numbers.
Jimmy G may turn out to be the second coming of Tom Brady, but remember he was also a record setting college QB, and a 2nd round pick who has been in the same system for three years, learning from some of the best coaches of the modern day, and watching Tom Brady prepare and execute. He was decently regarded by many teams coming out of college (compared to the 7th round pick who had never started in college, Cassell).
Brissett in turn is a rookie who barely broke 100 yards passing in his only full game. The coaches did a good job of playing to his strengths (designed QB runs, options, naked boots). Based on how McDaniels chose to use him, clearly they don't believe he is capable of running the same offense that they would run with Tom or Jimmy. Just look at the difference in the Pats offense between the 1st and 2nd halves against Miami. Brissett didn't make mistakes against Houston, and he executed the game plan that was designed for him. He was NOT asked to win the game himself though.
Conclusion: Aaron Rodgers or Steve Young playing well for the same team doesn't mean Favre or Montana weren't amazing QBs in their own rights.