Flanagan is an excellent bellwether of our defensive cohesion. His physical shortcomings mean he plays with very little margin for error in his positioning/predicting play. Thankfully that is far and away his strong suit, so when there is a reasonable amount of space for him to cover because he's got a clear role in a unit he looks tremendous, and you don't see him getting run ragged.
But the perfect storm created by Skrtel/Mane being introduced meant all the sudden he was up against a physical specimen and he felt responsible for twice the geography. That, coupled with being fed a couple tough passes in a congested midfield (at least partially occurring because others were trying to avoid using Skrtel, I'd imagine) and he had one of his bad games.
There is a strong argument against him ever being first choice in our team simply because a bad game can be so bad for a player with his physical limitations once his game becomes a series of physical contests, but you have to feel for a player who can look so classy around defenders on the same wavelength.
Makes you wish he'd been around in the early/mid 2000's to plug into those units. Or that Klopp is currently building a new one.