How was it painfully obvious at that point that he couldn't do it?  I would have probably pulled him before he was, but your definition of painfully obvious is different than mine.

What did seem painfully obvious was how bad Koy Detmer was in the USU game.  He looked like a high school player trying to play D-1 college football. 

My guess is that the coaches realized that Detmer is not the answer and that they needed to burn a redshirt as opposed to using him.