Ex-keeper van der Sar suggests the young Spaniard was a vital component in United's title win last season and feels he has so much more to offer as he continues to improve his game.
"It is unbelievable," van der Sar 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."
For breaking stories and all the great banter like us on Facebook: facebook.com/tribalfootball