The Dutchman, now working in the front office at Ajax, questioned De Gea's quality this time last year.
But he now says: "It's amazing how that boy has developed.
"He came from Spain and did not speak the language. England has different food, different tactics and a different way of training. Especially corners, free kicks and high balls are very different in the Spanish league.
"Physically, he has also grown tremendously."
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