Ansu Fati became Barcelona's youngest LaLiga goalscorer, but the Spanish champions had to settle for a 2-2 draw at Osasuna.
Fati, 16, scored his first goal for Barca six minutes after being sent on as a half-time substitute, to cancel out Roberto Torres' early strike for Osasuna.
Brazilian Arthur, who also claimed his first goal for the Blaugrana, put Barcelona ahead soon after, but Torres equalised from the penalty spot in the 81st minute.
Barcelona trio Lionel Messi, Luis Suarez and Ousmane Dembele were all absent through injury, while Segunda champions Osasuna now lie a point above the title holders after three matches.
Late goals from Sergio Leon and Morales secured Levante a 2-0 home win against Valladolid and lifted them up to third place in the table.
Leon broke the deadlock from close range in the 83rd minute and Morales made sure of the points in time added on.
Levante, who finished seven points above the relegation places last season, have won two of their opening three matches.
Joselu's first-half equaliser earned Alaves a point in a 1-1 draw at Getafe, who had taken the lead seven minutes earlier through Jorge Molina.
Real Betis moved out of the relegation zone after their first LaLiga victory of the season, 2-1 at home against bottom-of-the-table Leganes.
Martin Braithwaite gave Leganes the lead early in the second half, but Betis hit back with goals from Loren Moron (54) and £17.7million summer signing from Lyon Nabil Fekir (61).