Van Nistelrooy, a legend at United from his playing days, joined up to work under countryman Erik ten Hag this summer.
The ex-forward, who was manager at PSV Eindhoven, admits the lure of Old Trafford brought him back.
Speaking to Manutd.com, he said: “It's special to be back, to be fair. I mean, when United first came in with the opportunity to become assistant manager, it was fantastic.
“I never thought of it, but when it occurred, it was something that I couldn't refuse, with the role that I was going to fill. Of course, then (I was) flying into Manchester, (seeing) Carrington and Old Trafford and town and how much changed and how much is still the same. So it's special. It's very special to be back.”
“On a collective level, it's important to get the different game phases into the team, what is expected in the pressing or in the build-up or in the mid-block or in all the phases of the game,” he added on his approach.
“Like clear: what do we want? How do we want the execution to be and how do we translate it into the training pitch? I think we both are aligned with that and also with the manager, of course, and from there on you look to make an influence on individuals or units.
“In the end, when we're on one page, then we are definitely the strongest. That's our aim and it will take time, but I think we had a good start so far.”