I think our defence will improve as the players gain experience. Romero is the the leader with experience but VDV is still very much learning as is Udogie. Poro has dramatically improved form his start at Spurs. A lot is being asked of them and as you say the midfield is not working properly yet. Bisouma declined after his red card and his African tournament. I was beginning to think the malaria had scuppered him but he seems to be improving in recent games. I had a friend who got malaria and it never goes away every so often he finds himself ill and weak. Reports said the malaria he caught does not have lasting effects. I hope that is right. I thought Bisouma was going to be our midfield enforcer but that has not happened so far. I think you can see Vicario adapting and getting more robust. I think he was just shocked that he received no protection from the ref, and so was I.
Watching back parts of the game and focusing on their goals, while there are obvious defensive errors (Gigi beaten too easily at his near post, where have we heard that before?, Royal losing Salah etc.), what concerned me enormously is that every single Liverpool goal came with one of the CMs at least 20 yards away from the action and the other two doing absolutely nothing of note. The combinations change for each goal as well, which suggest that this isn't about individual players, it's about the collective not understanding what it is meant to be doing.
Goal 1 (Salah): All three CMs are located within 10 yards of each other. They are neither marking anyone nor putting pressure on the player with the ball.
Goal 2 (Robertson): Bentancur is 30 yards out of position, slowly jogging his way back. The other two are chasing the ball not the runners.
Goal 3 (Gakpo): Sarr is 40 yards out of position, the other two are again just drifting about aimlessly. Romero should be stronger in the air but he has Gakpo and Diaz to deal with at the same time, because no one is covering him.
Goal 4 (Elliot): Admittedly a top class strike, but he has all the time in the world to pick his spot. Bentancur makes a half arsed attempt to tackle, Bissouma just watches him line up the shot, and Sarr is once again marking thin air in the box.
It's a complete mess.