Barca's record-breaking start to the first half of the campaign, when they won 18 out of 19 games and only dropped two points in a 2-2 draw at home to Real Madrid, had many pundits already proclaiming them league champions.
The Catalans still have an 11-point lead over second-placed Atletico Madrid and an 18-point advantage over stuttering champions Real, who both play their games in hand on Sunday, but the first doubts have arisen at the Nou Camp this week.
On Wednesday, they were held to a 2-2 draw at home by Malaga in a King's Cup quarter-final first leg, conceding the equaliser against 10 men in the last minute.
On Friday, Barca were rocked by the news that long-standing keeper Victor Valdes had rejected contract renewal negotiations and would leave at the end of his current deal in 2014.
The defeat in San Sebastian was the first time this season they have failed to win in consecutive games since Vilanova took the helm in the close season but Pep Guardiola's former assistant sought to play down the result's significance.
"It was inevitable we would eventually lose a game," he told a post-match news conference.
"It puts us in our place because there is still a long way to go in La Liga. Although everyone believes we have been winning games easily, it isn't so. There are still a lot of difficult games ahead of us."
Next up for Barcelona is Thursday's King's Cup second leg at high-flying Malaga, who have also reached the Champions League last 16, and captain Carles Puyol appealed for calm.
"There is no need to get carried away because of one defeat," the Spain defender told reporters. "We need to go to Malaga and win. We think we are playing well."
Barca, who led 2-0 on Saturday, hit the woodwork twice and had chances to secure a win in the first half but the game turned after a red card for their defender Gerard Pique early in the second half.
Credit goes to Real Sociedad and their French coach Philippe Montanier for an aggressive and dynamic display that unsettled their visitors throughout the match.
Barca have now failed to win in their last three trips to Anoeta but Montanier, who has struggled to convince locals since joining in 2011, was not getting carried away with the win.
"The players fought for everything," he said. "We knew it would be tough but the players stayed strong mentally.
"Barca are a fantastic team, even with 10 players. They could play the whole championship with 10 players.
"There is no recipe for beating Barca. We won because we stayed compact, we worked hard, and we had a bit of luck." (Reporting by Mark Elkington; editing by Ken Ferris)
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