Splitting the CBs

One of the most crucial parts of football is positional cover. Meaning, if one player is pulled out of his position, another teammate must drop into that space to provide defensive cover. It's one of the fundamentals of team sports. And this relationship is perhaps best manifested, and certainly most crucially represented, by the CB-DM-CB triangle. 

The CBs are instructed to protect the goalmouth. To, no matter what, be the last two bodies between ball and goalkeeper. The DM, for all his on-field duties, at its core is the protectorate of the defence, of the CBs. When players ignore this duties, goals are conceded. 

Champions League gameweek 2, 2018/19. 

The ball is being moved out onto the right flank. Fabinho is currently positioned exactly where he should be. And Captain Henderson is dropping a tad deeper should something go wrong in these final minutes of the match. Nothing out of the ordinary, yet. 

Mertens's movement has dragged van Dijk entirely out of position. Both will soon be by the touchline. This is essentially the cue for Fabinho to turn his head, asses the defensive line, and drop into the holes that need plugging. Instead he, and Henderson a tad below him, are both caught ball watching. 

Napoli progress further into Liverpool's half. The entire left halfspace has opened up, now that van Dijk has been dragged out of position. There is space for Callejon to run into, space that Fabinho should be moving toward to stifle Napoli's attack. But again, he and Henderson are ball watching. While Gomez and Alexander-Arnold, the last two and the youngest two of Liverpool's back line, are left to defend the goalmouth. There is an attacker running off of each of them, and both, unsurprisingly, are ball watching. 

As Mertens passes the ball into Callejon's path, Fabinho is finally picking up his pace. Already too late as he will surely be beat for pace. Gomez, understandably, is ball watching, giving Insigne all the needed time to make any blind-side run that he desires. Alexander-Arnold is not doing enough to keep up with Insigne nor to eliminate Milik's cutback run. Henderson finally tuns his head to asses the defensive situation. As the next picture will show, the Liverpool captain does nothing with this information. 

Callejon is running into the penalty box from an entirely uncontested left halfspace. Fabinho and Robertson can't catch him. Gomez is trying to position himself to intercept the ball, but Insigne can make any blind-side run that he desires. Alexander-Arnold finally checks his shoulder for where the danger lurks, but he looks to the wrong side and afterward stops running altogether. 

In brief: van Dijk has been pulled out of position. No fault of his own, this will always happen to CBs several times in a match. There were two natural DMs in Liverpool's midfield trio that night: Fabinho and Henderson. Fabinho never dropped into van Dijk's position, despite being right by the left halfspace during the time of the attack and having multiple opportunities to do so. Henderson, despite this happening in the 90th minute, never made the effort to drop deeper and help the defence. Alexander-Arnold may be inexperienced, but still displayed horrid spatial awareness and defensive intuition. Gomez was left as the one-man defensive line.