Marcelo returned at left-back following an injury for the surprise loss.
"We lost but we must keep going," Marcelo said in the mixed zone.
"It hurts to lose because we played well. We conceded early on and then looked to stage a comeback, but it was not possible.
"We did not have time to think; they scored a goal and defended, waiting for the counterattack. That's no excuse, though.
"We need to play well until the final whistle. We have won some games while lacking something. We're trying to do everything possible to win but LaLiga is tough these days."