First of all a little story to tell you what kind of man we are talking about. It is January 9, 2008, and in an upstairs room at Manchester United's training ground five elderly men in smart blazers are struggling with their emotions in front of a hushed audience. It is the club's media day building up to the 50th anniversary of the Munich air disaster and Sir Bobby Charlton's polite smile does not hide the fact he is trembling as he takes his seat. Bill Foulkes is straight-backed and dignified but only a couple of questions have been asked before the tears appear in his eyes and he reaches for a glass of water.

In an adjacent room Wayne Rooney has agreed to offer a modern-day perspective of that seminal day when 23 people, including eight members of Sir Matt Busby's team, were killed in the wreckage of the burnt-out BEA Elizabethan. It is not his specialist subject but he handles the occasion with dignity and more eloquence than some people might imagine. But then Cristiano Ronaldo comes through the double doors and the mood is broken.

He is wearing a white suit jacket and ripped jeans, looking every bit the boy-band hunk, but it is very obvious he is in a bad mood. He begins by berating Karen Shotbolt, the club's press officer, because he is waiting for Rooney and the event has over-run. He is banging his watch with his hand, flapping his arms and gesturing in the way that Portuguese footballers usually reserve for fussy referees and, at first, he is so animated it appears as if it might be a wind-up.

When he flounces back through the doors, cursing loudly, it is very obvious he is being deadly serious. Rooney is professional enough to carry on with his tribute but the attention is no longer exclusively on him. Thirty seconds later Ronaldo appears again, first rapping his forefinger against the glass in the door, then opening it by a fraction and starting to whistle at Rooney in the way that a farmer beckons his sheepdog.

It was such an unpleasant scene the journalists decided not to write about it because we had been invited to the training ground to cover a far more important subject and, when you have sat with men as noble as Charlton, Foulkes, Albert Scanlon, Harry Gregg and Kenny Morgans and seen the hurt in their eyes, it felt incongruous to veer off-track. But coming away from Carrington that day it was difficult not to wonder what had become of the pimply teenager with the braces on his teeth who had been photographed, in his first few weeks as a United player, holding hands with his mother, Dolores, as they crossed a busy Manchester street.

The answer, of course, is that Ronaldo has fallen in love with his own reflection and, as United are currently finding out, that ego is in danger of spiralling out of control. Nor, sadly, is this story a one-off. One member of staff at Old Trafford reports being shocked by his rudeness when sorting out his travel arrangements for a club trip last season. And then there was last season's Football Writers' Association's annual dinner when, with barely any notice, its player of the year demanded that space was made for five of his friends to attend and that he would like them all to be on the top table with him. He got his way, as superstars often do, but the organisers were unimpressed, to say the least.

This is not to say that Ronaldo is all bad. He won a court case against the Sun earlier this week after it was reported that he had been fined for breaking club rules by using his phone during training: a story that was obvious baloney to anyone who has followed the player's career. Ronaldo, in many ways, is the consummate professional when it comes to improving himself on the pitch. He is not a man for nightclubs or raucous evenings out among the Manchester glitterati and there is something deeply impressive about the way he has come from his humble beginnings, growing up in Madeira in a house so small the washing machine was on the roof, to become the most penetrative attacking footballer in the world.

And yet United's more loyal and thoughtful supporters would by now be entitled to think it would be better for Sir Alex Ferguson and the Glazer family to end this shabby saga and let the previously unthinkable happen. To them, his constant prevaricating about his future, his flirting with the Spanish media and his apparent disregard for Manchester United, must smack of a man who has started to think he is bigger than the club.

His sound bites have become increasingly strategic, as if he thinks we cannot see what he is doing, yet nobody will have been surprised that the sweat had barely dried on his brow after Portugal's defeat by Germany on Thursday before he had re-iterated his desire to leave Old Trafford - just as Real Madrid had requested. United insist they will not allow themselves to be bullied into a corner but, when a player is acting like this and would so obviously be resentful and unsettled if he is denied the transfer he craves, the question should be: what is the point in keeping him?