I don't want to play an expert on Amorim's system (apparently almost everyone on here knows his brand of football now), but I do believe Mainoo is very flexible midfielder that can be tasked with different roles to play. The reason why I think he'll be a massive success is because (as far as I know - and I might be completely wrong here...) Amorim's 343 uses two man midfield with one focused strongly on defensive side of the game, and the other one more box-to-box, what I think suits our squad really well but only with Mainoo in that B2B role. I remember reading some quotes from United coaching staff that called Kobbie "a sponge for instructions", so I am really not worried about how he used to play for us.
Let me ask this question, do you think United Casemiro is same as Real Madrid Casemiro? A lot depends on the instructions and the role those players are asked to play, and how the team is set up in general. Put everyone forward and see Casemiro getting overrun. Give Mainoo some angles/passing options and see him progress the ball.
Alternatively, if dribbling skill is required for #10 in Amorim's system, maybe KB plays there? I stand by my initial comment, he'll be mint in Amorim's team, or any system that has some proper coaching behind it.