Where will he play? More importantly, what will happen to Valencia, Nani and Young?
Kagawa by all reports plays more centrally. Therfore he will either go in the the attacking pool of players with Rooney, Chico and Welbeck. Playing off a front man, whoever that is.
He may of course be seen as Giggs/Park replacement and play as a left winger who likes to cut in. This would be competition with Young. Nani and Valencia are both right wingers, so would continue there competition as per normal.
Finally he could play as Scholes replacement in the midfield, probably playing along side Carrick or maybe Jones. Picking the ball up further back on the pitch and passing around. i don't know whether he would be best suited for this however and see him been picked of a front man, or as the left winger.
We could go for a Barca style.
------------ Carrick
Nani -- Cleverly -- Kagawa -- Young
------------ Rooney
It gives us some good options, which is always nice.