If Kagawa hasn't earned his right to start, then has Welbeck? How about Young? Giggs? Hell, have Nani or Valencia earned it more? I'm including all these players because, guess what, Kagawa is still playing more on the wing.
Based on last season and this season so far, Kagawa has been better than all the players I mentioned above. That pretty much makes Kagawa our best winger, even though it's pretty far from his natural position(though I'm not gonna start that discussion again).
In the no.10 role(i.e his natural position), he's been very good overall since early March. He was brilliant/very good against Norwich, West Ham, Aston Villa, Sociedad and Leverkusen. He was average against Tottenham, and good against Everton.
Overall, Kagawa has been one of our best(probably the best) wingers from mid 2012 and up until now. He's also been very good in his natural position overall. The bottom line here is this: if Kagawa hasn't earned his spot, nor the right to be "defended", then we're really only left with Rooney. All other AMs and wingers are below par and their position in the starting XI can't be defended then.
( Seeing as I can't reply to anyone due to the personal rules I set before, I'm gonna say it here: I didn't include Januzaj in the list because I still feel it's too soon to draw a conclusion. Let's see after the season is done. The signs are very promising though. )
EDIT:
To prevent a new storm: no, I don't think Kagawa has been very good on the wing overall. I think his overall performance on the wing since he got here has been a weak 7 or a strong 6. Nothing to celebrate. But seeing as he's been our best winger overall during his time here(imo), we can't really throw him out of the starting XI either. Especially when he also can play behind the striker and do really well there.