Cristiano Ronaldo was so "obsessed" with football that Alex Ferguson used to order him off the training field as he honed his talents.
Ronaldo joined United from Sporting CP at the age of 18 and has gone on to become one of the all-time greats, winning the Ballon d'Or five times.
The Portugal captain sensationally returned to Old Trafford in August after prolific spells with Real Madrid and Juventus, becoming the LaLiga giants' record all-time goalscorer.
Ferguson was a father figure for Ronaldo following his move to United as a teenager, but the legendary former United manager had to send him home to save his legs early in his first spell at the club.
Ex-United midfielder Nicky Butt revealed in the foreword for new book 'Viva Ronaldo': "Ronaldo was to emerge as a genius but it wasn't just about his talents that took him right to the top.
"It is often said that certain players worked hard, stayed behind for some extra training or to practise their techniques, but with Cristiano it was total dedication, total commitment, and total hard work.
"He didn't have a God given physique, it wasn't given, he worked hard to achieve it. He actually came to us as a skinny, scrawny young kid, and worked hard to get that physique over many, many years.
"He had to reach a new level of physical strength because the Premier League demanded it, and he had plenty of opportunity to see at first-hand how physical and tough it can be when he was up against, even in training, the likes of Roy Keane and Jaap Stam.
"We would also see him work relentlessly on his free-kicks and he would tell us about something he had seen somewhere, and he wanted to perfect it, and he would work again relentlessly until he did.
"It all went beyond dedication, he was obsessed with football. You see it all the time when some players talk about their dedication and how much extra practise they put in.
"But often it is little more than a pretence, maybe a touch more time practising and then rush off to do something else they are keen on, or can't wait to go home and have something to eat, but rarely do you find someone totally committed 24/7 to the game, but Cristiano was one of those.
"He never had a girlfriend back then, or seemed that interested in any distractions such as that, he never had a wife and kids to rush back to see or to entertain, he lived alone.
"He was the last to leave the building. In fact, you couldn't get him off the training pitch as he worked relentlessly on his techniques, as we all walking off he was carrying a bag of balls to work on those techniques.
"We would be having lunch in the canteen or getting ready to go home and you could hear the voice of Sir Alex bellowing across the training fields at two or three o'clock in the afternoon, shouting at Cristiano that it was time to get off the pitch as 'we have a game in two days - enough now'!"