Through six games, Liverpool have conceded four goals, but 7.0 expected goals against. Having an elite keeper like Alisson in goal can help a team outperform their expected metrics, but having the 8th best defence is not a recipe for titles.
Manchester City and Liverpool are neck and neck in goal differential and expected goal differential. In a season where the title race looks incredibly tight, Liverpool's defence could be the deciding factor.
It's not entirely clear what is plaguing Liverpool's defence. They have an impressive stable of centre backs in Virgil Van Dijk, Joel Matip, Joe Gomez, and Ibrahim Konate, and Fabinho has cemented his place as one of the best defensive midfielders in the world. The main difference in this season's side is the absence of Georginio Wijnaldum.
The industrious midfielder was an integral part of Liverpool's midfield. Wijnaldum excelled in two crucial areas that could explain Liverpool's defensive decline.
The first is that he was nearly impossible to dispossess. With the near-universal adoption of the high press, having a press-resistant midfielder has become incredibly important to stifle opponents' attacks. The second area was his ability to cover ground. Defending is about being in the right position and few players covered as much ground as Wijnaldum.
Liverpool have the depth in the midfield to right these wrongs. However, it would have been a lot easier to replace Wijnaldum by simply giving him a contract extension. If Liverpool's defence doesn't improve, they'll have a hard time reclaiming the Premier League title.