The 22-year-old struggled to settle into English football following his move from Atletico Madrid, with Anders Lindegaard jostling with him for the number one spot.
However, the Dutchman, who spent the last six years of his career at Old Trafford, believes that the Spaniard has grown in confidence and physical stature during his time in the Premier League.
"It's is unbelievable," PA quotes Van der Sar as saying. "He came from different league, he was young. 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."
De Gea was named in the Premier League's team of the year last season.