Kagawa, afaik, plays wide left for Japan with Honda in the role everyone seems to say Kagawa can only be good in, behind the striker. I say, a really great player can play just about anywhere. The problem is the style of the team, utd aren't a great pass and move team.
Last year Giroud/Podolski/Cazorla linked up great, Giroud brought the running from the other two into the game, they played off Giroud rather than played for Giroud.
Kagawa is that kind of player, wants a striker to play off and other midfield players making runs into the box and players playing smartly to make space for himself or people for him to pass to, to run in to.
Utd don't play well, they don't play a Dortmund style short passing game. Giroud looked a fraction as effective if Gervinho played, and Podolski looked massively less effective if it was a player like Gervinho upfront instead of Giroud. He'd pass the ball to Gervinho, make the run and not get the ball, or Gervinho wouldn't be in a position to look for a pass from Podolski.
Kagawa just doesn't suit the team playing their kind of football, being played wide left or behind the striker doesn't matter. He may be better in the middle, he might not, we're not talking about worlds best behind the striker and forgets how to pass when played on the wing, it will be a small difference.
He needs games to be sharp, which he hasn't gotten in 18 months, and he needs better smarter movement from the players around him.