I'm behind on information it seems because honestly I only found out that Joao returned to Chelsea this season after seeing him play in this match 
Lol… maybe you don’t follow up the the transfer news that’s why you missed it because almost everyone who follows transfers new knows that Joao Felix has re joined Chelsea on a permanent transfer this time around and is now fully a Chelsea player. I don't think Chelsea's defense is as bad as they say, it's just that their performance is very unstable. This team needs a coach who is truly respected by the players, like Tuchel or Jose Mourinho in the past, but now many players feel confused because they often get a different coach every new season.
For now the defense is not perfect but they will definitely improve with time. They are just new to the system and it’s normal for players to feel a bit confused when there is changes in team play and instruction by the manager. All the players have talent and i believe that they will be stable with time.