Aritz Aduriz, 38, scored an 89th-minute overhead kick to steal all three points for the home side, despite Barcelona dominating possession.
Without Lionel Messi, who is working his way back from injury, Barca looked toothless going forward and only managed two shots on target.
It didn't help that Luis Suarez went off injured in the 37th minute, of course, leaving a front line of Antoine Griezmann, Ousmane Dembele and Rafinha.
Barca also started without Sergio Busquets, Ivan Rakitic and Artur in midfield.
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It was a hugely disappointing night for Griezmann and Dembele in particular, with the former making his league debut for Barcelona after completing a €120m move from Atletico Madrid.
Griezmann hasn't even met Messi properly yet but appeared to miss the Argentine, who Barca always struggle without.
The Spanish media haven't been very kind to the Frenchman and Dembele for their performances, which you can read below.
"Dembele 3/10 - Negative performance, added nothing in attack. His talent is inversely proportional to his understanding of the play.
"Griezmann 4/10 - Tried to be dangerous but needs to work more with his new teammates. Weak second half."
"Griezmann - Very quiet match on his debut. Better inside than outside. In moments he hindered Jordi Alba, didn't find his teammates and never seemed a threat. As a centre forward he brought spark. However, he didn't find the goal at any time. For now it shows an extreme shyness. He has a hard time understanding his teammates and is overwhelmed by tight defences."
"Griezmann - His lack of adaptation and rapport with his teammates was evident in several plays in which he did not understand anyone. If he asked for it to feet they gave it in space, and if he asked for the diagonal, they gave it to his feet."
Ouch. Griezmann will at least be given a few more games to prove his worth for Barcelona, whereas Dembele is living on borrowed time.
And with Neymar seemingly on his way back to the club, the young Frenchman ought to be concerned about what the future holds.News Now - Sport News