IMO Bill is probably “sticking with” both guys this year. He will plan to have Mac as the starter and do his best to prop up a good offensive situation around him, but will also leave the door cracked for Zappe builds on an impressive rookie showing and comes into minicamp and training camp looking sharp. Even then, Mac would probably still be the starter unless he looks bad in camp (which seems unlikely) but I do think Bill will keep an eye on both players. That good offensive situation benefits either guy. He may also lean on O’Brien as the QB Coach to give his own analysis of the capability of both players as camp goes on.
It’ll be interesting to see how things change as they presumably go back to an EP system that Mac already ran for a year, but Zappe only has experience with the bizarro Patricia-Judge offense.
Either way, I think it’s for the best as Zappe having a chance at the job means he pushes Mac to be his best.