Last year Josh primarily tailored the offense to Mac's strengths (lots of runs, etc.), just like they are doing with Zappe this game.
In preseason and early this year I got the impression that some combination of Patricia and BB decided to quit pulling back on the reins for Mac, try to install a new offense led by a rookie Offensive Coordinator, put in new starters at 4 of the OL positions, and really put things on Mac's shoulders to see if he could respond to a heavier burden. He struggled some, as did the OL (especially in preseason).
As you say our rookie OC is getting better at play calling. I'm hoping and expecting that Mac does well when he comes back now that the OL and play-calling has improved. The fact that Zappe looks as good as he does is just a big plus for the Pats, having a reliable back-up on the cheap is a real gift. And, if Zappe turns out to be the long term solution and outperforms Mac in the long term, well, then we've got two QBs on rookie contracts, not a bad problem to have.