Pro Bowl Player
I expect that by the end of the season BB will know if Mac is QBotF. But I expect what he does is likelybto be the same whether or not he thinks Mac is that guy.
I expect Bill will use the fifth year option, *and* draft (or potentially sign) the best QB he can get. Priority of getting a high quality QB prospect may be adjusted if Mac looks like he is that guy, but I still expect a prospect will be drafted to upgrade from Zappe. Mac will be competing for his job in training camp next year and the year after. Even if he remains starter next year he may end up sitting in year 5 if the guy drafted next year is good enough to play after one year of seasoning.
Pretty much the only thing I can see changing what happens would be BB deciding that Mac is not the guy, and cutting ties in the off-season. That means there’s a succession plan, and Mac is really not making the grade (whatever that might be). I don’t expect this is very likely however.
My expectation is that if Mac fails this season, BB will go back into the draft for a new QB. He might hold onto Mac for a while to see the new QB in training camp first and if he likes what he sees he will ship Mac off for like a 4th or 5th round pick if he can even get that much by then.