Somebody will always be free, so decide in advance who it is

No defensive block covers eleven attackers, and the teams that suffer least are the ones who pick their victim deliberately.

Ten outfield players cannot mark ten attackers and also protect the middle of the pitch. Everybody knows this and almost nobody says the consequence out loud, which is that every defensive structure is a decision about who to leave alone. Not a weakness. A decision.

Watch a well-drilled mid block against a back three and you will see it clearly. The far side wing back is free, permanently, by design. He can have the ball whenever the opponent is willing to spend two passes reaching him, and the whole shape is arranged so that by the time it gets there the block has slid and he is receiving facing his own goal with a full back arriving. That is a plan.

Compare that to a side that never made the choice. The block drifts, someone steps out because he feels the space, someone else covers for him, and the free player ends up being whoever happens to be unmarked when the ball arrives. Sometimes that is a wing back forty yards from goal. Sometimes it is a ten between your lines with his head up. You have taken the same risk without choosing where to take it, and that is the difference between conceding territory and conceding chances.

The interesting part is picking the victim. It should not automatically be the worst player. It should be the player whose best action does you least harm, which depends on the box more than the man. Leaving a poor crosser free is only clever if the opposition have nobody to attack the cross. Against a side with two tall forwards, I would rather leave a good passer free in front of the block than a mediocre winger free beside it.

That reasoning almost never appears in public analysis, where a free man is treated as an error. It is treated as an error because the camera shows the pass reaching him and the goal that follows, and does not show the four alternatives you were closing off.

If your team concedes and the pundit says a player was left unmarked, the useful question is whether the coach chose that or the shape chose it for him. Those are two different failures and they need two different fixes.