There are two "ideas" in the Kagawa debate that I always will find very amusing.
1) The idea that it's "just" Kagawa to blame. Had we signed Silva, Mata or Özil, then things would have looked so much different.
2) The idea that Rooney and RVP are too good to be dropped or played out of position, regardless.
1. Do people really think that if we signed Özil he'd get to play in the no.10 position under Moyes? No way. He'd be shunted out left, and when his level dropped, people would start writing bullshit like "well, I guess the strong, awesome PL defences are too much for him!" or "that's the difference between Spain and England!". There's an alternative universe out there where Kagawa stayed in Dortmund. In that very universe, this entire forum is wanking over him, wanting us to break the bank when the transfer window comes. The truth of the matter is that we'd need big changes in order to accomodate these small technicians and play so-called entertaining football.
2. What if we had 3 world class CBs? Would we then play 3-4-3 or 5-3-2? It's clear as day that the 4-4-2 with no proper link-up or gameplan where we just pray for our strikers to create magic out of nothing isn't doing us any favors under Moyes. The Kagawa transfer was meant as the start of something new. People who think that Fergie was an overly proud totalitarian who only did things his way, needs to get a reality check. Fergie was always good at adapting to the game, and he was inspired by others to change our play style to better fit Europe. Kagawa was signed for this very purpose, despite protests even from some of his players(Rooney for instance), who wanted us to stick to the old style.
Moyes has completely discarded Fergie's project, which he of course has the right to do, being the manager and all. But it's definitely not unfair to call him out on it, and question whether or not it is the right move.
The truth is, unless we let Kagawa get a good run of games in his favored position, then we haven't really tested him out properly. You might argue that Fergie never gave Kagawa this luxury either, but that's largely to blame on injuries and the fact that Kagawa needed to settle to the PL style first. Also, we've never lost a game where Kagawa, Rooney and RVP have started together. I think that's roughly 10 games. And then there's the Fergie effect. Kagawa looked really good under Fergie, even on the left. There was no reason to rush things.
The Fergie effect is gone and Kagawa's confidence is down in the drains. He barely gets playtime compared to the Fergie days, and when he does, he has to play for a team that clicks way less than before, and he's taken off the field even when he's playing good. In short, he's been dealt a shitty hand.
Our team wasn't perfect under Fergie, but we were far more threatening going forward, and much more fluid. Stats have shown that Moyes likes to attack with wingers/fullbacks much more than Fergie liked. Say what you want about Moyes: he has gone in the completely opposite direction of what Fergie to. And that has seriously affected Kagawa.