There are few things more sad than the visible signs of decline, especially when it concerns a sporting legend and especially when the legend in question is the only one who misses the point.
When Muhammad Ali lost a comeback bout to Trevor Berbick in 1981 it was as if the memories of the previous two decades, when Ali was the finest and most charismatic boxer of his generation, were tarnished forever.
Might the same be said of Ronaldo, the Brazil striker whose erratic career has baffled observers for 12 years? Was Ronaldo's wretched performance for Brazil during their match against Croatia in Berlin three nights ago tangible proof that he is now a footnote in history?
On Wednesday, Ronaldo was sent to a clinic in Frankfurt "complaining of dizziness and headaches". The Brazilian Football Federation issued a statement confirming that "no abnormality was found. Ronaldo is, therefore, in normal conditions for training."
Carlos Alberto Parreira, the Brazil head coach, has already confirmed that Ronaldo will start the match against Australia in Munich on Sunday, an announcement that has caused surprise among those of us who watched the player perform so lethargically that a flight home seems the only appropriate conclusion.
Brazil defeated Croatia 1-0 in spite of Ronaldo, rather than because of him.
He needs just one goal to surpass Pele as his country's leading World Cup goalscorer but he was substituted after 69 minutes, looking no more a part of the performance than the Croatian supporter who broke through the barriers to run on to the pitch. Television cameras cut to Ronaldo, sitting on the bench, looking as though his world had crumbled.
In some ways, it has. He seems to be all fame and nothing else.
It seems to sum up the Brazilian mentality that, in the wake of this victory, all the talk was of how one player failed to perform, conveniently forgetting that another player, Kaka, was the most impressive player in the whole of the first week of the World Cup.
Brazilians like to exaggerate. They like hype, but one Brazilian journalist friend of mine was clear in assessment of Ronaldo. "This was a man in the team on reputation, on what he has done in the past, rather than what he can do now and what he can bring to the team. He is not fit, he does not seem interested and he could do Brazil more harm than good."
Flashback to Paris, July 12 1998 and the news that Ronaldo suffered a fit just hours before the World Cup final against France. Ronaldo played that day, never looked comfortable and cut a forlorn figure as the final whistle sounded to confirm France's 3-0 victory.
Ronaldo had been impressive prior to that match, but his presence at the Stad de France that evening has baffled observers to this day.
Ronaldo redeemed himself in Yokohama, Japan, on June 30 2002, when his two goals ensured that Brazil won the World Cup final against Germany. The pain of 1998 was part of the happiness of 2002. Ronaldo had shown true greatness.
But Ronaldo has always been prone to injury, prone to gaining weight, prone to extra-curricular activities. He is a modern-day superstar, one whose fame transcends his sport and whose life story transcends his art.
It was once said that he became great at football because his buck-tooth grin made it impossible for him to attract women. He left Brazil as a teenager, joining PSV Eindhoven of the Netherlands and, later, Barcelona. He was an unused substitute in the
1994 World Cup final when just 17. France '98 was supposed to be "his" World Cup, but his date with destiny came in Yokohama four years later.
It appears, however, that WM2006 here in Germany is a World Cup too far.
Ronaldo scored four goals in France 98 and eight goals in Korea/Japan 2002. That means he is just two goals away from equalling the all-time goalscoring record by one man in the World Cup.
That is held by Gerd Muller, the German, who scored 14 goals (ten in Mexico 1970 and four in WM1974). And, like Ronaldo, Muller led a troubled existence, seemingly unable to deal with fame.
But Ronaldo has proved his critics wrong so many times. He went into the 2002 World Cup having barely played for the previous two years. It appeared that a knee injury was about to cost him the best years of his career. He bounced back, however, to play a significant role in Brazil winning the World Cup.
He secured a move to Real Madrid on the back of his performances in Korea and Japan but has, so far, spent more time fighting injuries and weight gain than scoring goals in La Liga.
Sad sights are everywhere at this World Cup. There was Diego Maradona in Hamburg, wearing an Argentina shirt and dancing like a drunken fan during his nation's victory against the Ivory Coast last Saturday. There was Michael Owen looking weak and unfit during England's victory against Paraguay in Frankfurt earlier that day.
But Maradona's decline is now part of the fabric of the beautiful game, while Owen will surely improve once he is over the psychological barrier brought about by that foot injury.
Where Ronaldo is concerned, this is sadness on a different level, because the World Cup needs him, yet the World Cup looks as if it is about to lose him.