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The Dutch forward established himself as one of the Premier League's top forwards at Old Trafford but controversially left in 2006 after an argument with young Ronaldo.
Van Nistelrooy allegedly told Ronaldo he had found a new dad in Carlos Queiroz, Sir Alex Ferguson's assistant, not long after the youngster's own father had died.
In a chat with Rio Ferdinand during Vibe with FIVE, the 44-year-old explained: "I know that I was wrong in that situation.
"I immediately walked into the dressing room and I sat next to him in the dressing room and apologised for my behaviour when I'd calmed down.
"That frustration is coming from somewhere and now when you're 44, you look back on things and you know when things are going on around you, sometimes it can make you frustrated.
"In that incident, it led to a discussion with Cristiano but of course, you need to realise and be big enough to acknowledge that."
"Ruud had that kind of ego: he wanted all the passes," he told Four Four Two.
"And sometimes, for the development of Cristiano Ronaldo and Wayne Rooney, it was difficult for the manager to deal with.
"Did Ruud make Ronaldo cry? Yeah, there were stories because I think they ended up in an argument at the moment when Cristiano's father had passed away, so it wasn't the right moment.
"Things like that happen when two players have lots of spirit, but I'm sure Ruud regrets some of his words."
Van Nistelrooy's inability to play with Ronaldo and a young Wayne Rooney was also cited as a reason for his departure.
Ferguson begun to build his team around Ronaldo and Rooney, believing they could become Red Devils legends.
Meanwhile, van Nistelrooy expressed his anger and claimed they couldn't lead United to glory.
"I had been thinking about it for a while and talking it through with my wife," he told Voetbal International.
"Two or three times every year, I would think to myself what a shame it was that it had ended like it did with Ferguson. I wanted to do something about it.
"My wife suggested I [send] Ferguson a text to see if he was willing to talk and when I sent him a message asking if I could ring him he replied to say: 'OK, call me at this date and time.'
"I rang him and he answered and said: 'OK, fine, go ahead.' I said I wanted to apologise for what had happened and he replied: 'Fine, I appreciate that, when we meet again it will be OK.' That was great to know."
Featured Image Credit: PA Images
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