#3 Fullbacks not offering enough

Like most coaches these days, Maurizio Sarri wants his fullbacks to play high up the pitch and offer an extra attacking option. However, they need to return to their positions while the team is defending.
This is an area that two Chelsea fullbacks are really struggling with. More often than not, both are caught out of position in counter-attacks. How Marcos Alonso has kept his place in the side is something most Chelsea fans are struggling to understand. Even Cesar Azpilicueta has lost pace, and it is becoming easy for oppositions wingers to beat him for pace.
This causes the centre-back pairing to go and defend in wider positions, which leave space open in the middle. They are also exposed to 1v1 situations regularly, which both David Luiz and Antonio Rudiger struggle to deal with.
Modern day football requires fast, athletic, and skilful fullbacks. There was a reason Pep Guardiola spent an enormous amount of money two years ago on the position. Chelsea should make that a priority in the summer.