It is too easy for a well drilled side to overcome zonal marking. I remember Dave Bassett as Wimbledon in the old 3rd division days summed it up simply. He said he went to watch the opposition and notice from the programme cover picture they defended zonally. He then instructed 2 players to attack the same zone - they scored twice. And that was almost 40 years ago!
Zonal marking has a place and has been mentioned can work if you have a goalie who commands his areas - we do not (as so few goalies do these days).
As with most things a mix seems to be best some zonal (e.g. near pot/corner of 6 yard box), some man marking and some blocking.
Why no one was ear marked to block or mark DeLigt was so amateurish - not only is the bloke about 6 ft 4, he scored only last week.
We need to be better drilled both defending and attacking set pieces. Look at Arsenal yesterday (and last week.. and the week before). Someone stands 'innocently' directly behind the goalie so if he back pedals the two will 'accidently' collide. Their best header, Gabriel makes a late run to the back post and the rest is history. No hit and hope involved - a clear training ground routine.