He's a technical possesion-based player in a hideous footballing side that cannot keep the ball and lacks ball carriers in midfield. McTominay and Casemiro just do not complement him, partly because of the tactics, and also because of their clear limitations. He needs quicker and more dynamic players around him.
The midfield needs to be rebuilt around Mainoo with players that will enhance his strengths. We have to stay patient and see what we come up with in the summer, but the likes of McTom and Casemiro are holding Mainoo and us back.