The Space Between Centre-Halves

Watched Neftchi's 3-0 win over Andijan last week. The scoreline tells you something, but the shape told me more.

Neftchi set up in a 4-2-3-1 against Andijan's 4-4-2 -6. The interesting detail was Andijan's two forwards. They kept drifting wide, trying to find space behind the full-backs. That left the centre of the pitch empty.

Neftchi's central midfielders, Djokic and Jovovic, had time to turn and pick passes. They found Odilov and Iskanderov between the lines, and the damage came from there -2. Andijan's forwards didn't fix the gap between the two banks of four. Once that space appeared, Neftchi used it for all three goals.

Andijan's shape made the half-space available. Neftchi just walked into it.

Question: When you see two forwards split wide, who should cover the central space they leave behind?