The gap that decides the game

Look at the first Milan goal against Napoli. Pulisic drifts inside from the left and the Napoli right-back follows him into the half-space. That leaves the entire flank open behind him.

Saelemaekers arrives from deep. Nobody tracks the run because the midfield line is ball-watching. The pass comes and the shot is simple.

When your full-back abandons his zone, someone must cover. If the holding midfielder is slow to shift across, that lane stays open all night.

Check the full-back first. The ball arrives second.

Question: When your team loses the ball high, who is responsible for covering the space behind the attacking full-back?