You caaaaalled?
Seriously though, even if Mata is the better player, I don't feel that the distance between the two is as big as people in here are suggesting. Judging by what I've seen from Mata in Chelsea, Spain, and United, I honestly don't think he's much better than Kagawa if we break down the different skills.
Kagawa is slightly faster, has better stamina, and better off-the-ball movement. I also think he's slightly better at dribbling. And while it's not an important skill to have for an AM, he is better at winning the ball back, both in the air and on the ground.
As far as passing, creativity, vision and technique goes, I think they're fairly equal.
What makes Mata better than Kagawa, is his insane effectivity and awesome set pieces. Seriously, Mata's awareness in the box and ability to always be at the right place at the right time, makes me think he could have been a world class striker, if he only had pace. His overall game for us hasn't been better than Kagawa's, but his stats are clearly superior. At the end of the day, football is about end product, and Mata has so far shown that he is better than Kagawa at this. It's evident when we compare the two players that Mata has always played in an advanced role, whereas Kagawa started out as a defensive midfielder.
Kagawa is a slightly better playmaker(offensive cogwheel, if you like), but Mata clearly has the superior end product. To top it off, he's PL proven(i.e. a God) and exceptionally likable. I can see why people prefer Mata over Kagawa then.
But the bottom line is that we're extremely lucky to have
both. We should do whatever we can to keep them. Seeing Kagawa go before the likes of Young, Valenica, Nani and even Welbeck, would be a disaster.