This is genuinely nuts. He's a promising young player, but one who's lacking in several fundamental abilities to play in midfield; pace, stamina, positional sense, security on the ball, etc. While there's an argument that he should be starting over Bruno at the moment, that's only because Bruno's form is atrocious, rather than Mainoo pulling up any trees.
He's got so much he needs to improve in order to be a success in the Premier League, both physically (e.g. fitness, pace) and technically (gives the ball away far too often, often caught out of position, etc). On top of that, he's generally quite lazy off the ball, and doesn't seem interested in pressing or recovery. First name on the team sheet is frankly ludicrous.