The system asks a lot of players in most positions, not just fullback. If one goes to shit, the whole team needs to go up a level to recover it.
Re fullback position: Walker is fucked & Zinchenko is shit. So we are left with a converted mid left & an irregular player at right back.
Add to that, the number of times Fernandinho either occupies the attacker out wide or covers for the centre backs, sometimes even in the line, thus providing an extra man, changing the position of the defence in general & it's not difficult to figure how it might go pear shaped with several missing.
