I agree, here are some interesting numbers.
In 10 years in the league, Cassel helped his teams win in 33 games out of 71 with an avg. of 0.464. with 95 TDs scored (combined) and with 105 turnovers (70 ints & 35 fumbles lost) We can all agree that both the Patriots and Chiefs were good, solid teams.
In two years, EJ Manuel helped his team win in 6 out 14 games, that is a 0.428 avg. with 19 TDs scored (combined) and 16 turnovers (12 ints & 4 fumbles lost)
I don't really see how he can be deemed the realiable starter, the safe bet as they say, when there's 10 year data that concludes that he is less than average as a starting NFL QB.
The good news regarding Cassel (and all QBs actually) is that he has shown improvements the last couple of practices, lets hopè he can continue this upward trend, I will support him fully is he's named the starter.