Well, it's not like he had a lot of work to do... All teams we played against parked the bus and had not challenged the defense at all. Just wait when we play against some quality opponent who will begin to send cross after cross into the bock to take advantage of Mascherano's height.
Well, the whole idea is to stop the other teams from doing that to us. If they do cross after cross in our box, something is wrong in midfield and on the wings. Mascherano is maybe the best at reading the game from our team, while Bartra is arguably the worst.
Our defence will never be good at resisting pressure, unlike Chelsea's for instance. But it can be very good at stoping izolated attacks with accuracy and good reading of the game. Which is what we need tbh. Defending set pieces is pretty much the first thing we need to master. Except that, we need to be excellent in 1 vs 1s and on the interceptions.