For some reason, our friends on the television love to show us possession statistics as if they're somehow meaningful. 58% possession is interpreted to mean that a team dominated the game, that they 'should have', according to TV broadcasts worldwide, won the game. This isn't true. I won't go into numbers right now, but I know that there have been several studies done that show that total possession has little or nothing to do with actually scoring/conceding goals (and thus winning games).
Why is this? There are a number of trivial, obvious reasons - the most commonly cited is that passing the ball around the back lines for ages does not exactly scream 'goals are forthcoming', despite giving a team a nice boost in possession. However, that sort of brand of possession football is actually reasonably effective in preventing goals - it's always been hard to score if one doesn't have the ball. So again, why isn't possession a reliable indicator of winning games?As far as I can tell, the real reason we can't use possession as a crutch is the threat of the counterattack. Simply speaking, the longer a team has the ball on the attack, the more defenders they're facing as they flood back to get between the ball and the goal. This means that the attacking side has to commit more players to the fray or their foray into enemy territory will likely die down rather quickly. The only place the extra men can come from is the defensive lines - by strengthening the attack, a team is temporarily ruining its defensive shape.
Attacks, of course, are not guaranteed to succeed by any stretch of the imagination. The transition between attacking and defensive play has frequently been cited as the most important phase of the game; it is the time that a team is at its most vulnerable. AC Milan were undone this weekend by driving too many forward and then losing the ball. Twice, Cesena scored on simply counterattacks. If Cesena were dominating possession and leaving AC Milan in a defensive shell, there's no way they would have scored in the way that they did.
Jose Mourinho recognised the danger driving forward posed to his team's organisation in the second leg of last year's Champions League semifinals, going so far as to instruct 10-man Inter to deliberately lose the ball against Barcelona, confident that they could keep the Catalan giants at bay as long as they maintained their positions. While this is an extreme strategy, it's very clear that having the ball for too long is extremely dangerous, and that fast breaks are a much safer way of scoring goals than having the entire team move forward and expose the defence.
Counter-attacking is becoming more and more prevalent in football - how many goals do you see scored 'against the run of play' these days? While it's nice to put pressure on the opposition and generate chances, one has to be mindful that the longer a team is entrenched in the opposition half, the easier it is for the other side to score a goal if they achieve a breakout. Transitions, in other words, get much more difficult the more time you've spent on the ball in enemy territory. And that, ladies and gentlemen, is the curse of possession.