Rabanal, now sporting director of Independiente della Valle, was on the staff of Van Nistelrooy at PSV Eindhoven.
The United great was named assistant coach to manager Erik ten Hag for this season.
Rabanal told The Sun: “Ruud had a great connection with the players and it was great to see the relationship he had with everyone, from top to bottom.
“Although he was an elite footballer, a world legend, he is such a humble guy and so calm and unaffected by difficult moments.
"That doesn’t mean he wasn’t demanding but working with him was a pleasure. He was so infectious and created a really positive environment.
“Perhaps another person with that standing in football would have a massive ego. But he just knows how to talk to people and enthuse them.
“What really impressed me was he was available for anything, at any time for anyone and when he spoke, it was with real feeling, and I know he touched the players’ hearts.”
He added: “Ruud is going to contribute so much. Working with him was a master’s degree in elite football.”