Babb, who also represented the Republic of Ireland, played for the Portuguese side in 2000, when Ronaldo was 15, before leaving in 2002, before the former Manchester United man made his debut.
Ronaldo only played for one season in the Sporting first team before moving to Old Trafford for £12.75m as a replacement for David Beckham, who had left to join the three-time Ballon d’Or winner’s future club Real Madrid.
In an interview with Sky Sports’ ‘The Fantasy Football Club’, Babb revealed how Ronaldo’s tricks in training left him so embarrassed that he decided to “forearm chop” the then teenager to teach him a lesson.
"[Ronaldo] was really skinny then and he had this big mop of hair – a bit like Steve McManaman back then," Babb said. "We're doing a training session and it's attack versus defence. So this little kid runs at me. He steps over twice, three times, goes past me, sticks it in the top bin.
"All the lads are like 'Babbsy he's had you off' and all that, in Portuguese. So the next time, he's coming at me: step over one way, step over the next, he shoots past me – so I forearm chop him. I knock him out.
"So Ronaldo's on the floor, and I'm like 'nunca mais' – Portuguese for 'never more' or something like that. He's looking at me and he doesn't know what has happened. The training ground's gone quiet.
“Needless to say, he went down the other side and never came back."