Sheraldo Becker's 33rd-minute goal consigned Barcelona to their second league defeat of the season, leaving the Catalan giants six points clear of second-placed Real Madrid, having played an extra match.

Lewandowski's early disallowed strike led to furious debate over whether the Polish forward had been offside or not.

"It was the wrong decision, clearly, it's clear, but we have to accept it," Flick told the club's streaming platform Barca One.

"The referee, it was not his fault, we are humans, we make mistakes and today was a big mistake."

Barcelona were without teenage star Lamine Yamal after he suffered a blow to his ankle in the Champions League win at Red Star Belgrade midweek and badly missed his spark.

The Catalans failed to score in a match for the first time this season across all competitions as Real Sociedad, eighth, held firm, while Barcelona did not manage a single shot on target.

"The responsibility for this defeat is on us, we are responsible, we can play much better," admitted Flick.

"It's a tough place to go, where La Real push a lot... we have to learn from our mistakes and that's it," Barcelona midfielder Pedri told Movistar.

"Normally we score goals but today we weren't accurate, I think it will be a one-off, and in the following games we we'll be more precise.

"I think the team is working well and (not scoring) is not something that worries me."

Playmaker Dani Olmo started on the bench while defender Pau Cubarsi played in a mask after suffering a cut which needed stitches on his face in Serbia.

The Catalans have struggled in San Sebastian in the past and were given another uncomfortable night in the Basque country.

Lewandowski thought he had fired Barcelona in front after 13 minutes when he reacted well to a blocked shot to fire home, but the goal was ruled out following a VAR review.

Referee Guillermo Cuadra Fernandez was told Lewandowski was offside, although the forward did not appear to be in front of Real Sociedad defender Nayef Aguerd.

The semi-automated offside technology used in La Liga indicated the front of Lewandowski's boot was off, but Spanish media furiously debated whether the technology had misinterpreted Aguerd's heel as part of the veteran striker's boot.

- La Real dominant -

With Barcelona's strike chalked out, the hosts took control and tested visiting goalkeeper Inaki Pena several times.

Takefusa Kubo's low drive stung Pena's fingers after a fine dribble inside from the right flank by the Japan international.

Becker fired Real Sociedad ahead with a cool low finish after Luka Sucic nodded the ball into his path.

Flick brought on Dani Olmo for the ineffective Frenkie de Jong at half-time but Barcelona remained second best.

Becker screwed a shot wide after Jules Kounde did just enough to put off the Real Sociedad winger, and Ander Barrenetxea also squeezed an effort wide from a tight angle after bursting into the area.

Despite scoring 40 goals in their first 12 league matches Barcelona were unable to find the net or even create any significant danger in the final stages.

"We really wanted to win at home and there was no better team to do it against," said Real Sociedad midfielder Brais Mendez.