Galloway, Driver, Smith, Ward... all have had big days against our defense. Those guys were MM's primary responsibility (#1 WR).
On the flipside, there is not a single #2 WR that has given us problems this year.
I understand that it is not as easy as saying that MM is responsible for all of the #1 WR and Freddy the #2, but the Saints _do_ play a lot of man coverage. In those situations, it is that easy.
The difference between Thomas and McKenzie is the fact that when a play is made against Freddy, he is usually at least on the screen when the WR makes the catch (and he makes the tackle). So you say... "there's Freddy running with that WR, he got beat". When MM's assignment makes a play, McKenzie is usually 15 yards away from the play and off the screen.