1. It's not about leading 3-0. It's about playing quick, technical, fluid, attacking-minded football.
2. It's a huuuuuuge difference between playing better when the team plays well and becoming a catalyst. When the team delivers a solid 7 out of 10 performance or higher, players like Kagawa are there to give it that extra push that turns the performance into an 8 or a 9. Does players like Valencia, Young or Welbeck do that? No.
I get it: you don't like players like Kagawa. It's been clear from the beginning. I hardly remember any usernames in here, but I remember yours. I have no issue with your philosophy, but when you suggest things like "United is too big for Kagawa" or "Kagawa is just a mediocre players who needs to be carried by the team", then you're talking bullshit. The Dortmund team that he was an important part of would have walked the league this year if they were in the Prem, with Kagawa being one of the standout performers. It's not about United being too big. It's about us playing absolutely shite football that highlights individualistic moments of brilliance, rather than actual good football.
When the team plays good football and Kagawa is on the field, then he'll be one of the top 3 performers 80-90% of the time. This was the case for Dortmund. This is the case for Japan. And this has been the case for us, even under Moyes. It's painful to admit, but Kagawa's "failure"(he's still our best winger, as sad as that is) just proves that we're lacking as a team. Put Kagawa in pretty much any other top team except for a team managed by Mourinho, and he's an absolute star. We suck these days, and thus Kagawa struggles. His off-the-ball movement becomes pointless, because none of the other players move with him. His crosses and passes become meaningless, because there aren't enough movement in the box, and the ones who receive the ball misses their chances anyways(just look at that youtube video).
Kagawa doesn't have an impressive longshot, nor set pieces. He's not particularly fast, and let's not even mention his physique. He's also never been a dribbler, nor is he comfortable out wide. How on earth do you expect a player like this to thrive in the current United team? Hell, even the likes of Silva and Iniesta would only be a shadow of themselves if we signed them.