I think Fergie will plan to use Kagawa in a variety of different positions. English teams are in a different situation to continental teams in that the style of play we face in the domestic league requires a very different approach to that which we will face in Europe. We need players who have the attributes to be able to play in different roles in different systems and Kagawa fits the bill.
In the 4-2-3-1 we usually play (2 midfielders, 2 wingers, 1 forward dropping deep off the frontman) Kagawa could play anywhere in that 3, much in the same way that Mata has for Chelsea since Di Matteo came in and changed to that formation. Squad rotation will assure everyone gets games and his ability to roam from a central position or to come inside off either flank makes him very useful as a starter and as an option off the bench.
In European games and big domestic games where we might want to be a bit cagier and use 4-3-3 I can see him being used as the most offensively-minded of the midfield 3 (or on the flanks of the front 3 if our designated wingers are out). In those games where we sit back I think a player like Kagawa who could receive the ball in our own half and burst forward quickly past a player or two at speed before picking a forward pass would be a huge boost to our counter-attacking play.
edit - oops, the discussion seems to have come round to whether other people were in for him, this bit is in reply to the people debating 'who he'd replace'