If Powe and Davis are not back (or only one of them), I don't think you can use the lack of practice as an excuse not to play Garnett a few minutes.
I understand not playing him both nights, and not playing more than 20 or so minutes...but right now they are desperate for bodies (particularly tall ones who know the defensive rotations).
In a perfect world, Powe, Davis, and Garnett will all be back for Friday's game, with Garnett playing 15-20 minutes, and then on Saturday, Garnett rests, and then he is able to build up some stamina over the next few games after that.