Buffalo has a better roster and was playing at home, they should have won, all else being equal. The simple fact of losing doesn't concern me. We have a rookie QB who has been good but is definitely a work in progress. Starting tackle is out. The usual stuff that can reasonably contribute to losing.
What concerns me is that all this stuff that supposedly sets Belichick apart from everybody else was TOTALLY ABSENT. The defense was not smart or physical, in fact it seemed to just check out of the game after one quarter and let the Bills walk all over them with no resistance. There were no clever schematic twists that slowed down the opposing QB, even for just a drive or two. You've got a dumb facemask penalty, blown special teams coverage. All the supposed hallmarks of a "Belichick coached team" were a mirage. They didn't know what to do and they didn't fight. For this team to go anywhere they had to play mistake free football with strong defense, instead they played dumb football with no defense.
In short, the game was a coaching failure more than anything. This isn't some knee jerk BELICHICK FRAUD FIRE HIM post, but it is what it is. You don't lose by 40 as a 10-7 team because your roster is THAT BAD, you lose because you were soft, sloppy, unprepared - all the things Belichick is supposed to prevent. They go out, I dunno, 31-20 and I'm totally on board with "they just need to bolster the roster, Mac needs a year under his belt, etc." but not after this.