The pair are good friends after their time playing for Spain's youth teams growing up.
Mata told El Pais of De Gea's United career: "The first year was difficult . He had to put on kilos of muscle and struggled to adapt.
"He suffered in the media, but has a tough mentality. He is quiet, yet has a very strong mind. That has continued to now.
"Last season they awarded him as the best player on the team and this of course is what we're seeing from him."
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