The Red Devils brought in their former striker, and ex-PSV Eindhoven boss, Van Nistelrooy to aid countryman Erik ten Hag.
While Ten Hag is the club’s manager, Van Nistelrooy and Rene Hake have added experience as his assistants this summer.
Defender Jonny Evans commented: "It's always good to freshen up. Ruud coming back has been great, especially for myself."
"I grew up sort of idolizing Ruud and the way he used to play. I got a little bit of chance when I was in the youth team, to see him play.
"When he was coming to the end of his United career, I might have trained with him once or twice, when he was coming back from injury.
"When he was nearing the end of his time at United, I might have had the chance to train with him once or twice, when he was recovering from an injury.
"So it's good to have someone of that caliber and respect in the squad."
Goalkeeper Tom Heaton added: "Me and Ruud spent a bit of time together.
"I was always his preferred keeper for shooting practice.
"He's been absolutely brilliant since he joined, and his attention to detail is exceptional.
"You could be mistaken for thinking he's just a United legend coming in as a figurehead.
"But his meticulousness - and I've attended a few meetings where I've heard him speak - has been remarkable.
"He's the best finisher I've ever seen. I've seen some brilliant finishers, but I've not seen anyone better in my career."