Marc Guiu had only been on the pitch for a few seconds when he sent in the game’s only goal in the 80th minute, earning himself a dream debut.
The forward was freed out on the left by Joao Felix and outwitted the visitors’ goalkeeper Unai Simon, who got a hand on the ball but could not stop it from going into the goal.
Barcelona climbed to third in the table ahead of next weekend’s El Clasico against league leaders Real Madrid, who are one point ahead.
Before that, Barcelona will face Shakhtar Donetsk in the Champions League on Wednesday.