Mustafi is worth every penny of what we paid for him in todays market. Full international, young for a defender and is far from poor.
Defending starts from the front. You cant just buy a couple of CB’s and everythings sorted. Our open game in general allows the oppo to counter us effectively. This is why 75mil for VvD is laughable for me - Pool will still be opened up because they play a similar exspansive game.
Of course you’d rather a good defender rather than an average one, but the system and the players in front of the defenders can be just as important.
We won the FA cup final with a defence we shouldn’t have won with, all because the team as a whole played like a unit.