A good coach can turn average defenders into a goddamn good backline, with strict tactical directions for keeping the play small, efficient marking, and backup marking, disrupting opposing build up play if he study'd their playstyle well.
However, place the same defenders, in a very open space with a greater number of attackers running at you, its goddamn hard, knowing how to controll space, be aware of the positional situation, and then anticipate, is by far the hardest ability to master for a defender.
Chiellini, is incredibly good at it, so was tiago silva.