The Royal Spanish Football Federation (RFEF) confirmed the unexpected coaching change on Tuesday, bringing an end to Moreno's brief stint in charge of the 2010 World Cup winners.
Moreno took over in a caretaker capacity in March and assumed the head coach role on a full-time basis in June.
Luis Enrique had stepped down as his young daughter, Xana, had developed bone cancer. She died in August.
His former assistant, Moreno, won seven and drew two of the nine matches he oversaw in total, leading to a sense of shock when it became evident that Monday's Euro 2020 qualifier against Romania was to be his last game.
"In truth it is a little surprising, we all got a little surprise," Barca defender Pique told Mundo Deportivo.
"I'm not there, I haven't talked to my [Barcelona] team-mates because they played on Monday and they didn't train [Tuesday]."
"I am very happy that Luis Enrique returns to training, that is the best news of all," he continued.
"Regarding the relationship between Luis Enrique and Robert Moreno, I have no idea what happened."
Pique has been out of the Spain side since retiring from international football following the 2018 World Cup in Russia.