It was a free kick from Andrea Pirlo, well struck, and Filippo Inzaghi made a run in its general direction. That much was intentional. But he didn't strike the ball. The ball struck him. It hit him round about the shoulder or upper arm. No, that doesn't make it handball. The process was entirely inadvertent. If anything, he was trying to get out of the way. But the result of this little swerving half-duck was to propel the ball into the net; and the game changed for ever as a result.

You can, if you like, say that Milan deserved it on the balance of play, but I'm not so sure that's right. Liverpool had the better of the first half, in terms of shots on goal, and if Milan were better in the second half, that was because the goal had changed everything. Well, you can say that Milan were the better side on the night and that their second goal was a thing of perfection, but the game had changed because Milan had scored by accident.

That accident makes them the best team in Europe: champions, monarchs of all they survey. Eight months of striving by 32 of the finest teams on the Continent, preceded by a season's striving by a few hundred more, and it all comes down to a chap being accidentally hit by the ball.

And that's luck. The ball might have rebounded high, or wide, or straight at a defender. It might have missed Inzaghi altogether and been comfortably saved. It might any bloody thing you want, but the fact is that Milan are the best team in Europe by accident. Take nothing away from their victory, as football people like to say, generally meaning the exact opposite. The point is that luck is an irrefragable aspect of football.

It evens itself over the long haul. No one wins the Premiership by means of that kind of luck. You can have bad luck with injuries, as Chelsea did with Petr Cech this season, but that sort of thing also levels itself out over a season. Manchester United had an injury crisis of their own. In football, luck can change a game, it can't really change a season.

So if you want to rule luck out of the equation in football, the one thing you must do is to make sure that big prizes are never awarded on the events of a single match. And that is precisely what football does not do. Football goes to great lengths to do the exact opposite: to devise competitions that are decided by means of one great occasion.

Of the four trophies available to a Premiership club in Europe, three are decided by a single match. There are two great prizes available to a European national side, and both the World Cup and the European Championship are decided by a one-off fixture. The great prize can go to a side who, at the crucial moment, get lucky.

This fantastically high value placed on simple luck is more or less unique to football. Sure, you can have flukes in all sports, but there is no other sport in which a fluke can change the destiny of a match and a championship so often and so drastically. You can win a point on a net-cord in tennis, but that doesn't often happen on championship point. No match changes for ever because the ball landed this side, or the other side.

You can get lucky bounces in rugby. But it is very, very seldom that a lucky bounce decides the destiny of a match, still less a trophy. In both codes of rugby, there is a disconcerting tendency for the stronger side to win and the weaker side to lose. Golf, yes, a capricious bounce can take a ball into the water or, alternatively, not into the water. But over 72 holes, it's unlikely that a single freakish moment will decide a major championship.

In snooker, you can get some spectacular flukes: potting the ball from an escape shot, or potting the ball at one end after missing at the other. That can decide a frame, but over, say, 35 frames, a fluke isn't going to make all that much difference. You can lose your wicket unluckily in cricket, be on the wrong end of a decision or deflect a wide one into your stumps. But you have invariably got to take 20 wickets to win a Test match and it's seldom, if ever, that defeat comes down to bad luck.

In horse racing you can have bad luck in running and, as a result, you can lose the hugest of prizes. You can lose the Derby because you got caught in traffic in the first two furlongs. But avoiding such things is the jockey's job. You very seldom win or lose a great race by means of a freak.

Athletics, rowing, combat sports, judged sports such as gymnastics, skating, diving: no. You can't win the Olympic 100 metres with a fluke. Most sports are carefully framed to minimise the assistance of luck, to make a level playing field on which excellence will prevail and the best man, the better team, will win.

Only in football does luck play so huge a part. Only in football are the greatest of all prizes so often decided by means of chance. And yet football is the most popular game on earth.

Now I ask you this: is that a coincidence? Or is the possibility of luck an essential part of the game? Is it the high importance of luck that makes football so phenomenally attractive to so many million people?

In football, to win with a sprinkling of luck, or with a soaking of luck for that matter, is a wonderful thing. The little team so often beat the big team they have 25 attempts on goal but, as luck would have it, they can't hit the target and your brave lads break away, the centre half slips, the shot hits the post and bounces in off the goalkeeper's back. And then you argue about who deserved it and who didn't deserve it. But deserve ain't worth doodlysquat. It says 1-0 in the paper and that's an end to the matter.

Always, in every match, there is the possibility that luck will play a part, and perhaps a colossal part.

Milan scored a goal by accident and the goal is the highest-value currency in all sport. A lucky run, a lucky try, a lucky chip-in all these things have their place, but they very, very seldom decide the destiny of the great prizes. Nothing in sport is so lucky as a lucky goal in football.

And it seems to me that the random awarding of luck is a vital part of the game. We savour luck's cruelty, luck's injustice, the clear fact that God doesn't love us, and then we are intoxicated by luck's extraordinary benevolence; by the fact that we have been singled out and blessed in this extraordinary fashion. In football, we are luck's playthings. And that, clearly, is the way we like it.

(Simon Barnes writes for The Times)