Hmmm.... this is beginning to sound as if BW could use the week off IF Coach can afford to do so. But Kelly will only know that once he sees Book on the opposing field as a first time starter.
If Wimbush can play, but is gimpy, I'd start Book and see. I might have a quick hook, but in some ways this would be the most relaxed coaching situation about replacing QBs one could get at ND. Folks expecting BW to rest; folks not knowing what to expect from Book. Folks (mostly) understanding if Kelly has to pull him. Folks (mostly) understanding if Kelly needs to rotate them to give BWs ankle rests and Book's mind clearing time.
BUT if it's also clear that BW CAN'T play, do you really go with Van Gorder as back-up, or do you get the rookie ready?
p.s. I can easily be totally wrong, but Book seems to me to have a natural QB game-situation presence. Eye-tests can be WAY off, but that's what my eyes see when I see Book operating the team --- intuition said: this guy's a player.