João Mário is the type of player that gets underrated, he's very good on the ball, he presses well, his corners are fantastic, he can create chances and most importantly he gives a lot of balance and calm to the team by doing the small things right. He's a very smart player and I can see why Mourinho would be interested. His problem is that when he gets close to goal his shooting and general decision making is terrible.
Good player who's good enough to be starting for Inter even though it seems like he lost his spot, but he'd be someone United could buy for some depth in midfield, not to be a starter. There are far bigger problems in this United team that need fixing and João Mário wouldn't really solve anything imo.