As I already said, back post man, and you mark areas now, not men. Just modern defending, hence all the $#@! defenders.
It's been fun splitting hairs in the context of this game and single incident.
the back postman is not a direct threat and is covered by the wide defenders, the other two defenders have neutralized the primary threats, Bonucci was not in a position to intercept the back post man anyway, a long ball to the other side would give anyone extra reaction time, bonucci is useless on one on time, he cannot defend, he knows that and he avoids doing so. It is not a coincidence the same things happens to his cover zone all the time, he is less than a man less, he is a liability. if he wasn't even there the real defenders would perceive the void and would position themselves to cover it, with a CB there, they naturally give him a zone to cover, this zone always is our weakest spot, we know it, he opposition knows it too and they exploit it !