The former Ajax midfielder has signed permanently for Girona from Manchester United.
Jongkind told Sport: "I've known him since he was ten years old, as a 'little boy'.
"He was in the academy and I was working there.
"Donny van de Beek is a player with a lot of physical background, intelligence in the game - especially at the beginning - in the facet called 'third man', a principle that Johan Cruyff more or less invented.
"When we did the physical endurance tests at the academy he was always in the top five . Aerobically speaking, his condition is good. He has never been a 'monster' because of that.
"He is a boy who wants to win, to give his all. He runs a lot, he always plays with one or two touches, so his functional technique is very good. He understands the game very well."
Jongkind also said of Van de Beek's time with United: "It's difficult because there are many interests. There is no patience with anyone, you have to have immediate performance . I don't know exactly how he left Everton and how he joined United, but maybe he needed a little more time. More minutes. But if they don't give you that time, there is no chance to recover your level.
"It would be because of the pressure that Ten Hag and United in general still have. And in a club like Girona he will have more stability and could reach his maximum level. It will be a challenge for him anyway, but he will be in a team that will fit because of his game."