It was one of those impromptu conversations about football that make Monday mornings bearable. This guy stopped me in the Rio street, as he has done a few times before. I don't even know his name, but I respect his knowledge and opinion, and now he unloaded one on me that left me worried.
"I think," he said, "I've detected the first signs of the decline of Lionel Messi."
This is not what I wanted to hear. I have to declare an interest. I have acted almost as if I own Messi ever since I was lucky enough to see him in the South American Under-20 Championships in Colombia nearly eight years ago.
He turned up as an unknown, a 17 year old runt of the litter who had played one friendly for the Barcelona first team. And as soon as he picked up possession it was clear that we were in the presence of something special.
The little darting changes of direction, the sudden kick on acceleration, the vision of options around him, the unassuming joy at his own talent - the characteristics which have gone on to mesmerise the world were all on show back in early 2005, and I had the immense good fortune to see it first. As far as I am concerned, Messi is never coming down from the awesome standards he has set for himself.
But my friend thinks differently. There is no Brazilian anti-Messi agenda in his analysis, just a close study of Barcelona's early season form. He is also anxious to point out that Messi remains the best player in the world and is likely to be so for some time. But he fears that in the future we might be seeing less of his individual genius, dribbling his way through entire defences and making opponents collide into each other like cartoon villains.
I don't see enough European football to have any authoritative view on Messi's form so far this season. But my initial reaction was to dismiss the idea out of hand. After all, Messi is only 25, the age when, according to conventional wisdom, he should be just coming into his prime.
True, Diego Maradona was also 25 when he hit his peak with a succession of extraordinary displays in the 1986 World Cup. He remained a force for another few years, but the Aztec stadium in 1986 was the scene of his finest few hours. Of course, were it not for a life off the field that was spinning out of control, Maradona may have been able to sustain his golden years for longer.
But then there is Pele, similarly amazingly talented but psychologically very different. There has never been a shred of self-destructive behaviour to undermine Pele's career. But he too reached his peak level of performance at a relatively early stage. By his own account, his best performance came in Lisbon in the second leg of the 1962 World Club Championship, when he ran amok against Benfica. He was a few days short of his 22nd birthday.
There is a clear irony in the career of Pele. Internationally he is most remembered for his exploits in the World Cup. But the competition never really saw him at his best. When he first appeared in 1958 he was exuberant but raw, and had yet to fill out physically.
Four years later in Chile he was much more like the finished article. The goal he scored against Mexico in Brazil's opening game is the work of a football force of nature. But he was injured in the next match and played no further part in the tournament. That year could have been what 1986 was to Maradona.
Pele, of course, was brutally kicked out of the 1966 World Cup before rounding off his international career with a beautiful swansong four years later in that same Aztec stadium that was later to be graced by Maradona.
The 1970 Pele was still a genius. He had extraordinary vision and the capacity to size up the situation in a split second and choose the correct option.
Some of his moves - the attempted goal from his own half against Czechoslovakia, the dummy and dribble round the Uruguay goalkeeper, the passes that set up goals for Jairzinho against England and Carlos Alberto against Italy - will be drooled over for as long as football is played. But in athletic terms the 1970 Pele was past his best, heavier and slower than the younger version. He would no longer charge through a defence all on his own, the ball bouncing around his feet like an eager but obedient puppy.
This is the kind of dynamic that my friend is predicting for Messi: that as the years go by his capacity for individual acts of magic will be reduced. He will dose himself more, choose his moment and operate more as the fulcrum in a collective context.
My reaction is that it is still too early in the season - let alone too early in a career - to jump to such definitive conclusions. What my friend is witnessing could just be a phase, the result of carrying a few too many early season pounds.
Even so, though, I am very glad that he gave me his opinion. Not only did it make Monday morning more interesting, it also served as an important reminder. In the long-term battle between athleticism and time, there is only one winner. Whether or not it has already started, the decline of Messi will take place one day. Enjoy him while he lasts.
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