McKenzie is not a typical slot receiver but is a motion WR who opposing players cannot keep up with, one of few players on Bills offense who is able to make space, who did a lot of different things becoming puzzles for opposing offenses making them spend resources in preparation work and deciding which players who are active. When a team doesn't we get results as in Pat(hetic)s game where he erupts on field correctly making defense gurus look unprepared.
Unfortunately he cannot ca$h the check by his mouth (which I am sure disappointed him in free agency this year) and he needs to spend more time with jugs machine and preparation to get to the next level. No one else on team can do his role. When he is "on" he makes team much more dangerous; when he "off" the team needs to find some other way to exploit holes.