The Telegraph says ref Howard Webb warned City about the behaviour of their ball-boys after Ronaldo had the ball hurled at him.
Webb stepped in following an incident late in the first half when a ball-boy threw the ball at, rather than to, Ronaldo as he was about to take a corner for Manchester United during their 1-0 victory at the City of Manchester Stadium.
The Portugal winger - later sent off for a second bookable offence - earned the admiration of the match officials for not reacting to the aggressive gesture.
Linesman Peter Kirkup was closest to the incident but it was also spotted by Webb, who noticed that it was followed by a round of applause and a cheer from City fans crowded around the corner flag.
Webb delegated fourth official Lee Mason to speak to City officials at half-time and their subsequent warning to the ball-boys was heeded in the second half.