He drops deep when the ball doesn't reach him. So he gets back to get it, to try to do something.
He also makes NO effort to position when he is playing between the lines, just waits for the ball to move within 10-15 yards of his area before becoming involved. Messi is great at drawing defenders and creating space for his team
*when he has the ball.* When he is without the ball, he is invisible. He doesn't make runs in between the lines, and rarely makes runs to get the ball behind the back line.
When he drops to the midfield to get involved, that's when he gets marked more closely. Mostly, like other Barça midfielders with their back to goal, he plays wall passes with a midfielder when he has someone breathing on his back. Sometimes they'll try to 1-2 up the middle, but because Messi leaves the front empty, it's essentially 2v8 at that point against a well organized defense, and they lose the ball or dump it to the wing (where there is now no advantage, because the entire defense is compressed back to defend against the run up the middle).
If Messi is to drop to the middle, then other players need to be given the freedom to attack the empty space to give him targets. Iniesta does this well when he is not played on the wing. Pedro sometimes attacks that space on the diagonal, and Alexis does it well, but has no final product. Cesc can, but I didn't see it vs Milan.
