Mainoo is far from a progressive passer, and is neither a high-volume passer. He has excellent technical ability, and is very young so there is a good chance that he will become a great passing midfielder, but right now he is just a very good prospect with immense potential who mostly keeps it very simple (backwards or sideway passing). He has the tools to become very good at it though, and has high dribbling abilities that most good passing midfielders lack. But Bruno is the only progressive passer we have and we do not have a single high volume passer in our team (the likes of Carrick and Scholes were both progressive and high volume).
I am afraid that a midfield of Mainoo-Ugarte loses the midfield battle against any half decent team, and even Casemiro who is far from a great passer is miles better at passing than both. Someone like peak Carrick or Alonso would have been perfect for us, and perfect for Mainoo.