The Portugal international has spent the second-half of this season on-loan at West Ham.
“I'd like him to stay in England, it would be better for him," Mourinho told Record.
“He's started a good process and it would be right to complete it. It's a process through which he can establish himself in English football. Staying in the Premier League would be good for his growth.
“Joao Mario at United? Why not! West Ham is a historic, important club that wants to grow. Joao just had to adapt.
“He's someone who thinks and acts quickly, he's recovering intensity and aggressiveness, I've liked him more in every game."