The 22-year-old has today joined Ligue 1 side Lyon in a £16m deal.
"He is potentially a very good player," said Mourinho. "When Mr Van Gaal decided to buy him he did well.
"He knew him from the national team, the player was very young in the (2014) World Cup and had flashes of top quality.
"He didn't succeed in this 18 months, but he is very young and it is important for the club to keep control of his talent. We all wish he plays very well at Lyon. Why not come back one day because everyone likes him?"