So players are allowed to develop. McKenzie has developed since his rookie year.
Basically, this season his "numerous other muffs" are the horrible fumble in the Colts game, which deservedly saw him benched today,
and
a muff against the Dolphins
McDermott has stressed that he wants to see the punt end with the ball in our guy's hands, so I'm not sure you can put that on McKenzie and not coaching.
He was 6th in Y/R on kickoffs and 8th in Y/R on punts before this week, so I don't think he can be misjudging punts as often as you think.
Stevenson had a muff today.