Defour, the injured Standard Liege midfielder, revealed this week that Ferguson had written to him.
Van Nistelrooy later revealed: "When the deal broke down, Sir Alex was in Spain.
"I telephoned him to tell him what had happened and he was really surprised. He told me not to worry and said that whatever was wrong would be fine.
"He assured me I would still become a United player. He kept in contact every four or five weeks. He would phone me at home and has stayed in touch throughout my rehabilitation.
"When he came to see me that was the thing that did it for me. Imagine such a big club, such a big manager, who has had so much success, to show real warmth."