He's also facing vanilla defense. Not the disguised coverages, exotic line stunts and blitzes that you would see in the regular season. A guy who's been in the league for several years should at least be able to dink and dunk up and down the field in the pre-season. The fact that he hasn't and was terrible in his one regular season performance says a lot.
Pert of the issue with replacing him is cost. After the Donald deal and if we want to add any late free agents - you can't be throwing big money (Teddy Bridgewater) at a free agent back up. If they got anyone from the Jets it would be McCown. He's career backup, smart, and can learn McVay's system. No way the Jets part way with Bridgewater at this point. McCown is likely too expensive as well.
Brandon Allen makes the most sense, but not so sure about him either.