That corner goal highlighted two things imo.
1. we only had two good aerial players on the pitch- Varane and McT. The rest are mediocre in that sense.
2. There is no organisation or leadership what so ever. How come Eriksen marked Haaland?
Zonal marking. The old 'big lads take big lads' man-for-man system was pretty easy to abuse because it was too easy to take the opposition's best defensive headers out of the game.
With zonal marking, the best headers of the ball are always in the positions which require the most defending.
Now, the obvious problem with that is that if you're marking an area, the attackers are going to be able to get a run on you and should have an advantage if the delivery is right.
So...what you do is, ask the players who are not good defensive headers/smaller to block the opponents runs.
What happened on Sunday was that McT was in the key defensive position and Haaland was the target. Eriksen should have been blocking Haaland but it was a pathetic attempt. Just as teams have blockers for attackers, teams have blockers/distracters for defenders...so Gundogan was occupying McT, Eriksen didn't do his job, the delivery was great and Haaland was able to get up high and score.
I played for many years at CB/CDM and was always the main (or one of the main) defenders/attackers of a set-piece. I hated man-for-man because it's nearly impossible to prevent a good header of the ball getting a run on you from a standing start. Plus, opponents would block you off. I would much rather be given an area to defend.