That they do very little progressive passing is not in dispute. It is because of how our team plays, how Mainoo naturally plays, and also because of how Mainoo is coached or directed.
United often only had one good player able to pass/dribble his way out of the press, Mainoo. So if Mainoo wasn't picking up the ball from the midfield, United would go long or pass to the wings (winger or fullback) or give the ball to his partner who are more gungho. We know why McTominay is in the 10th percentile. He was one of those players that didn't have that ability so didn't like receiving the ball in those kind of situations. The other midfielders were not afraid of getting the ball in those situations, but were not as successful for different reasons. Casemiro for lack of ability, Eriksen for lack of legs, and even Amrabat was a little faster than Eriksen, but a little less ball playing ability. Why Mainoo couldn't best McTominay in that stat doesn't suggest he can't do it, it suggests that he didn't take charge enough in that phase of buildup. When others got the ball, they couldn't feed Mainoo enough, which would not be a problem with another player that has similar ability on the ball.
I think you guys are understating how United's lack of technical quality both in midfield, and defense contributed to Mainoo's progressive passing stats. Mainoo is at an age where you shouldn't write off his ability to develop this because all it takes is direction. He easily has the tools to be excellent in that area, which I believe is what is most important here. Give him partners with enough legs and ability on the ball and that stat will reflect better on Mainoo (if he doesn't have a donkey coach in Southgate), and it will not seem like it's a task that you need a 80th-90th percentile player to control matches alongside him. Why? Because his midfield partner will be able to pass him the ball if he doesn't constantly go towards the ball. Also because that new player will be able to beat the press, it will make the defensive organization less settled giving him more passing options for volume of progressive passes or it would mean the defense backs off which means more passing between the midfield partners (more touches for Mainoo).
I don't know how you guys can watch last season, discuss our midfield problems and then just take the statistics at face value for the upcoming seasons. This is a discussion about potential midfield partners for Mainoo, not a discussion on the past season where things stay the same, both because of incoming transfers and because Mainoo is a malleable 18 year old talent.