The 22-year-old was part of a Villarreal defence that conceded just 35 goals in 38 league matches last season.
"He's a very young boy coming from a completely different environment, but you could see what he is at this moment," Mourinho told British media.
"This was the first time he has worn a shirt like the Manchester United shirt. He can't speak English and can only communicate with the ones who are French-speakers or Spanish.
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"I wanted to give him more minutes because he needs to play more than others. He needs to feel it and start communicating."
The former Chelsea manager also likened the Ivory Coast international to defender Raphael Varane, whom he coached at Real Madrid.
"I will give him everything to try to make him better and better, like I did with other central defenders I've had in my hands," Mourinho added.
(Reporting by Nivedita Shankar in Bengaluru; Editing by Peter Rutherford)
