De Gea has produced several impressive performances this season, and pulled off a magnificent save to deny Emanuele Giaccherini against Sunderland last weekend with the Black Cats 1-0 up.
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"It is unbelievable," the ex-goalkeeper, now marketing director at Ajax, told reporters at the Leaders in Football Conference. "He came from different league, he was young. He was 20 when he came.
"He came from Spain, he didn't speak the language. There is different food, different tactics, different methods of training.
"Corners and free-kicks, balls in the box - that is different to the Spanish league. It was normal he could have some troubles in the beginning.
"I think the way he turned the corner from January onwards has been a real benefit for United and one of the two or three reasons they won the league.
"His physique is better. He worked really well with [former goalkeeping coach] Eric Steele and the physical coaches and that is the way you improve.
"He is still so young but already experienced. Hopefully he will get a chance in the national team also."