Put it another way I’d rate the areas of the team as a collective like this
Keeper - 7.5
Defence - 5
Midfield - 6
Attack - 8
This is mental though.
I'd actually agree with you in the sense that individual defensive performances were fine, very good in fact, but the defense collectively found itself in very difficult positions. That part isn't mental, it's that you put that down to collective defending, as though a defensive unit defends independent of the rest of the team. No. The TEAM defends. The team, including the attack is to blame for the amount of very high risk situations we find ourselves in defensively. The fact that often it is just a defensive unit defending IS the problem. This isn't american football.
This split between individual performance and the collective weakness of the team is because the team's structure was flawed. That structure is the entire team. Nunez can't control the center of a pitch, that's not his game, we don't have firmino. Gakpo was great at that last season, and then was poor for most of this, playing inconsistently in every sense of it. Salah was never a great presser, but fell off massively this season, he simply doesn't have the legs to contribute to defense and can't play the position he is now in the system we play. The midfield is meant to press high and contributed with more goals and more ideas, but they also gave up the ball too often, and struggled to cover a very weak flank down the right. The right back inverts but we aren't careful enough on the ball. Our play was lopsided, broadly. Our forwards weren't close enough to each other enough. Our midfield has some good players, but no one who is fast or athletic.
All of that shit puts your defense in bad situations, to where you can say "jesus this team leaks goals" but also "hey, Quansah is a revelation, VVD is back to his best..." and you give them a 6.5, 7 across the board.
But you blame the defense for that discrepancy.
When you have goal after goal where the midfield, which isn't athletic or fast, gets bypassed and our three defenders have to turn to run 3v3 or 4v4 straight at our goal as fast as they can, it doesn't matter what sort of drilling or telepathic understanding they have. A system of play is supposed to limit the amount of times that happens, and that's on the whole team. That shit happens sometimes anyway, and the defense then deal with it a decent percentage of the time? That's some great defending. No one can deal with it a lot though.