The former striker says he treasured his time working with then manager Sir Alex Ferguson.
He told La Gazzetta dello Sport: "When he signed. A surprise: austere but affectionate, attentive, a father figure accustomed to treating young players like jewels, protecting them and encouraging them to grow as people and as footballers, with very clear values and rules. We found each other because I had ambition and education, my father had raised me in a precise way: if I slipped up he would put me in line immediately.
"In training? Incredible. A different sport. Beastly speed and brutal intensity. You know the phrase, 'You play like you train'? Well, my dad always told me that, and there at United it was like that, but multiplied by a thousand. I quickly understood what I had to do. I had talent, I had to use it to earn the respect and trust of those monsters.
"Physically, I was behind, I had to get by with technique and intelligence, and therefore think of a new speed. Before the ball arrived, I had to already know what to do with it.
"Otherwise, Gary Neville or Nemanja Vidic would have been there to wake me up. Certain beatings... There's an anecdote that captures my mental state at the time. Roy Keane, in his autobiography, recounted how once in training he scolded a young Italian for not passing him the ball, and the boy responded by glaring at him. 'If he'd said anything to me, I would have hit him. He remained silent, but his eyes spoke volumes, he was telling me to go f*** myself. I thought about going and shaking his hand,' he wrote.
"That boy was me. And I don't remember the episode at all: I was obviously in a competitive trance, in training! My determination to make a career was absolute."