Sir Alex Ferguson let Cristiano Ronaldo get kicked by Roy Keane at Man United

Sir Alex Ferguson's plan to toughen up Cristiano Ronaldo involving letting established players like Roy Keane and Rio Ferdinand kick him in training, according to a former Manchester United coach.

Ronaldo arrived at Old Trafford from Sporting Lisbon as a teenager in 2003 before leaving the club for Real Madrid six years later as one of the best players in the world.

And former Manchester United goalkeeping coach Tony Coton has revealed how Ferguson devised a plan to get the best out of the winger.

"One flaw was his habit of falling to the floor like a rag doll every time he lost a tackle," Coton wrote in his new book "There To Be Shot At."

"Sir Alex embarked on a mission to toughen him up. Members of the coaching staff were encouraged to turn a blind eye if they saw Ronaldo being fouled in practice matches and small-sided games.

"These games were intense and no prisoners were taken. Poor Ronaldo didn't know what hit him as teammates he'd been sharing a joke with him gave him the kind of treatment that was no laughing matter.

"When a free kick was not forthcoming, he'd throw his hands up in outrage and sit on the turf muttering Portuguese curses while the game continued around him.

"Toughened pros like Roy Keane, Rio Ferdinand would lambast him for being soft. It was tough love and slowly but surely, the message began to seep into our winger's consciousness."