The scouts in the NFL can measure height, weight and arm strength, and such things. But they fail in scouting QBs because QB is NOT PLAYED on the Field. It is played between the QBs EARs.
Cassel has no missing athletic talent. He has ideal size, a strong NFL arm, and running ability. Bu that is not what makes a QB. He has demonstrated some things that some never display.
Now for ther things that happen between the QBs Ears:
a) He has POISE.
b) He has leadership ability.
c) He can process a Game-plan.
d) He can function as a QB at NFL speed.
e) He makes good decisions.
f) He avoids turnovers.
Kevin O'Connel has the physical attributes.
He has demonstrated exactly ZERO of these other items. Maybe he will; and he will become a NFL QB. Maybe he won't, like most, and he will fail, as a NFL QB.
In any case Cassel had produced some evidence like his play in the regular season Miami game in 2006, and pre-seasons that he had some of these extra attributes.
Kevin has displayed none as yet. Kevin is at least another year away from being a NFL #2 QB, that can be called upon.
As for his status as #2 last year, BB realized that if he lost another QB, the season was done, so he might as well use it to instruct KOC, IMHO.