Full-backs and the space nobody watched

Sassuolo 1-1 Milan. Look at the 68th minute. Milan had just taken the lead through Rabiot, and Sassuolo won a corner. The goal came from the second phase, not the first ball.

What caught my eye was the space behind Milan's left side. Estupiñán had been replaced by Modrić at half time. Milan lost their natural width on that flank. When the corner was cleared, Sassuolo recycled it quickly, and Milan's midfield could not slide across in time. Odenthal arrived late into the box, unmarked.

The passing lane that disappeared was Milan's ability to push out wide and close the second ball. Without it, Sassuolo kept the attack alive.

Watch who is covering the space behind the full-back after the first clearance.

Question: When your team defends a corner, do you look at the first ball or the players waiting outside the box?