"When he is building new teams he is willing to do that [be ruthless]. That is all credit to him. The club is the most important thing.
"That is what he always said. It is his most-repeated sentence. He proves that. You move on. I moved on to Madrid and had four great years there. He moved the club further onwards. It is fair enough.
"I didn't find it hard to deal with. I accepted it. I am grateful for five great years here and it gave me another opportunity to be part of Real Madrid for a good while and be successful there. It opened up a door and I moved on. Thank God, that was a successful period as well."
Sir Alex took charge of United when van Nistelrooy was just 10 years old. Since then, Ruud has forged one of the most successful professional careers of the modern era and now, aged 36, is enjoying retirement.
And still Sir Alex goes on.
"It is incredible," he said. "It is difficult to get your head around. Twenty-six seasons!
"Ten years in football is an awful long time for a manager to be at a club. He is going on and on. In three or four years we will be talking about 30 years. It is out of this world.
"But he's doing it. As he said, he has outlived death. He is outliving everyone."
Ruud was speaking to ManUtd.com, Manchester Evening News and PA Sport.