Bartra, formerly of Barcelona, believes Zidane has benefited from working with Castilla for 18 months.
He told AS: "With the continuity, the coach knows better the players, and it also gives more confidence to the young in the long term.
"The elite is day to day, but at the management level, you have to look at the long term and things are going well. Zidane gives confidence to the majority of players and it makes everyone perform well.
"The confidence of the coach is the key for the player to give it back and he even gives it to the young players."