McCollum has outplayed mrazek so far this year, and the wings need the win. They know what they have in mrazek even if he is in a slump. This will be McCollum's last chance to prove he is worth something. And if he performs well that leaves us with two solid prospect goalies, one can be traded in a package for something.
That has nothing to do with it. McCollum is done with the Wings. He'll never make the team as anything more than a third or fourth backup. What they're doing is sending Mrazek down to play some games while the Wings have 4 days off. Precisely because Mrazek is the future.