There is a difference between being a flop and being wasted. Kagawa has been forced out to an unfavored position the wing during his time at United thus far. If Moyes continues to use the 442 the same will happen to Mata, although I suppose Mata would do a better job on the left than Kagawa.
You're right, stupid tabloids overusing that term has rot my brains.
But again, Mata is less of a "gamble" per se, than Kagawa, because Mata has Premier League experience while Kagawa didn't. Every time I watch United, one big issue that they have is that they lack imagination going forward, their Plan A is mostly hoping for Valencia to cross which he fails at most of the time, and Plan B... well... they don't have much up their sleeves, don't they. Here's hoping Mata fixes that problem.
Speaking of which, I hope deadweights like Young and Valencia leave United as well. As much as I care less for United than I used to, I still watch them from time to time but watching them has been painful because they're so boring.