Garappolo doesn't have to be ****ty. All he has to be is ordinary, and that brings the Pats down to the level of the rest of the AFC East. I think all the AFC East teams have good enough defenses to deal with a backup QB.
I watched the Pats/NO preseason game, and I think that's what the OP was referring to in his opening post. Garappolo did not look very good despite playing the entire first half against the Saints D, not exactly a unit filled with great defenders .... and the Saints were playing their scrubs almost the entire first half. He looked like what he is: a pretty ordinary backup QB. In fact, the announcers, who were a Pats broadcast team that apparently spent time observing the team at TC, said that Garappolo hadn't looked very good in recent practices.
NE didn't have any of their starting RBs, WRs, or Gronk. I don't put a lick of stock into his performance.