A teenage Ronaldo arrived at United in 2003 with bags of ability. That was apparent to see right from the outset.
However, it took him a few years until he matured and became one of the best players in the world.
Fletcher thinks Smith's training ground drill is one of the major reasons why he went from being a frustrating player to one of the best players ever.
Fletcher, who is currently the technical director at United, told the fascinating story during an appearance on BeINSPORTS in 2020.
Richards Keys asked Fletcher: "Many described Ronaldo as a circus act in the early days. What changed? Or who changed him?"
View Fletcher's answer below...
“You could see this charismatic kid, you could see the ability, you could see the drive to become the best player in the world,” Fletcher started.
“But he frustrated the life out of everybody. Everyone thought if he could learn…
“Eventually Walter Smith came in and decided not to give fouls in training.
“In general, I think the sole purpose was for Ronaldo. So when Ronaldo’s doing his skills, not passing the ball and taking the mick, the lads were fouling him.
“Before, the first six months of the season, there’d be fouls given as you’d expect, and Walter said: ‘No, no fouls’.
“For two weeks Ronaldo was tearing his hair out and he was going bananas. Open season. Hit him.
“What happens after a few weeks? Ronaldo starts moving the ball because he’s sick of getting kicked.
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“Now he starts playing one or two-touch and running, now he starts getting goals.
“Now it starts clicking in his head: ‘I’m starting to score more goals, now I’m starting to make an impact in games’.
“That was the beginning of Ronaldo’s transition.”
Ronaldo may not have become one of football's greatest ever players without Smith.