Torres, who plays for Paris Saint-Germain, has been troubled by minor discomfort in his left ankle and, despite training in recent days, was released from the squad to prioritise his recovery, the RFEF said.
Coach Luis de la Fuente called up Espi in his place, rewarding the Real Madrid youngster after he scored five goals and set up two more in Spain Under-21s' 13-0 rout of San Marino on Thursday.
"Due to the persistence of the symptoms, the decision was made to withdraw him from the squad to prioritise his recovery," the federation said in a statement, adding its medical staff had remained in constant contact with PSG throughout the camp.
Spain host the Czech Republic in Oviedo on Saturday before travelling to face Croatia on October 6th.