Okay, first of all, I think Bill messed this up in a few ways. If Mac wasn't 100%, you have 3 options:
A) play Zappe, rest Mac
B) start Zappe, have sub-100% Mac available as a backup or to take a series here or there, working his way back.
C) start Mac anyway, have Zappe there as a backup or to split time.
Bill chose C, which is my least favorite option, but he knows more than me. But if you choose C, and Mac plays badly, you have to bite the bullet and wait for the right time to replace him, rather than right after he throws a bad pick. Pulling him when he did, it's hard for that not to turn into a confidence crisis and QB controversy.
My preferred answer is B. This gives Mac a little work that can become as much as you want/need him to play on his way back, but Zappe taking the reps and doing the game planning gives you a better read on him, and it becomes a win/neutral prop for Mac: if he takes some snaps and plays well, that's great, he's on his way back. If he doesn't, well, he's not 100% and didn't have the reps and game planning of a starter. We'll see in a week. But you give everyone involved an out.