He has to be a ton better on offense, and their is no speculative about it. Casilla is a damn solid MI, Mastro is not. You can hide a poor fielder at RF/LF/1B and even 3B. You absolutely can't hide one for SS/2B/CF. For a pitch to contact team like the Twins you really can't be rolling out bad defenders at key positions. Look at it this way. If they try it for a week or two and it fails, as you seem to think it will, that will end our discussions on the subject and we'll all be free to move on to all the more pressing issues in the Universe. Yea I think most would agree that chances of success are pretty low.
Ok hopefully better than one in a million, in any case I don't see what the harm is. The comment about the defensive dropoff to Casilla is a good one, but I doubt he will be back anyway, this might be a comparison to Dozier as he is the one who may be losing at bats next year if this does work out.