One problem is to treat the stuff he does with the ball as a kind of nice optional extra, rather than the foundation of our entire style in attack and in defence. A guy who passes as well as he does changes the way teams shape up to attack you, which changes how you defend, usually into something he's better at than he is at one-v-ones with Mateta - namely, anyone else noticed how since he's come back our offside trap works again?
Subtle game this, sometimes, small things can have big repercussions on the rest of the game.