Good things first, we got 3 points, and we won from behind second time this season (I think), which is a good sign of mental toughness improvement, too bad it is the WC break. Improvements in the details, De Vrij's and Skrinar's yellow actually were both crucial, more midfield interception in the second half due to how Alanta plays the game, and we were also exploiting the spaces differently since they were doing whole field man marking and no zonal at all, can't list them all; we didn't control the match, but I would say we did the right things in the detail along with some luck. Most important we were LESS wasteful.
I think there are ways to improve the defence without switching up the play too much, e.g. more set piece defending exercises & drills, I feel like this has always been one of our weak links, even last year or with Conte, we just didn't get exposed too much, but you still see it. e.g. when we played against Liverpool at home our defence was tremendous, except when it comes to set pieces, we looked extremely inferior.
I also think that how aggressive our WB determines the frequency of our goal conceding risks occurring, and how well our defence plan adapt the opponent.
I don't question the potential quality we have in our defence, else the game against Barca will be very different. I also lean towards it is mental/focusing thing instead of the set up in general.