Here is another Hayden Fry quote that is appropriate for the situation.
"(Having a proven winner returning at quarterback) makes you feel very confident about your offense, ... But the thing that offsets that confidence is the fact that as a coach you're always scared to death that your number one quarterback, who is really outstanding, is going to become injured. Normally, the drop-off to number two is pretty big because they are so good that it's pretty hard for the number two to fill their shoes." Hayden Fry
So there is not a huge drop off from Casey to Chandler, Chandler got reps last week.
Sounds to me like Diaco, Patterson and Cummings made some good decisions! I too hope Casey gets well, heck I'm from Monroe, my whole family went to Masuk! In the meantime we should feel confident that Diaco talked Chandler into coming back and also that he got significant reps in the BYU game!