Mourinho replaced Van Gaal as Manchester United manager in 2016.
Pereira is currently being managed by Marcelino Garcia Toral during his loan spell at Valencia.
Discussing his current and former coaches, the 21-year-old said: “They're two great managers, but Mourinho is more relaxed off the pitch.
“Of course there is discipline and you have to be on time but players are more relaxed. They feel good.
“There's less pressure because Mourinho takes it off the players and puts it on himself. During the preseason in Los Angeles, he told us that we could do whatever we wanted after training.
“He trusted us to rest and relax. To do what we wanted to do. There were a lot more meetings under Van Gaal."