Gerardo Martino's men lost for the second time in a week and, although they remain top of La Liga, the Argentine coach now needs to strengthen his squad in the transfer windowIt can hardly be described as a crisis. Barcelona sits top of La Liga with an amazing 40 points from a possible 45 and is through to the second round of the Champions League as the group winner. But two losses in the space of five days have taken the gloss off the Catalans' superb season start. Under the surface, the cracks are beginning to appear - and now Gerardo Martino must move to fill them by signing in January.
Defeat at Ajax in midweek had been put down to the club's considerable injury list, as a number of players missed out with muscle problems. The 2-1 loss in Amsterdam had been a freak result, they said. No reason for alarm.
But Martino was left furious by Barca's poor performance in the Dutch capital and insisted his team would put things right against Athletic. The Argentine boss had told his players exactly what he felt about their performance on Tuesday and was confident they would react positively at San Mames to show that what happened on Tuesday was nothing more than an accident.
Lightning, however, struck twice. Barca's goal on Tuesday had come via a fortuitous penalty; on Sunday the Catalans again failed to score from open play and rarely looked like doing so. Neymar, so promising earlier this term, was expected to grow in the absence of the injured Lionel Messi. Instead, he has shrunk and, like the rest of his teammates, seems to struggle against strong and physical opponents.
The Brazilian was fouled outside the area by Ander Iturraspe at 0-0 and Barca appealed for a red card for the Athletic midfielder. But it was only yellow and the Catalans were furious. "The referee didn't do his job properly - it was a red card," Sergio Busquets claimed afterwards. Neymar also came close to scoring in the first half with a rasping drive pushed over by Gorka Iraizoz, but failed to make the desired impact when Barca needed him most.
Martino will hope to put that right in January, but it is not the only concern for the 51-year-old coach. In midfield, Xavi looked tired and again struggled in a high-tempo game, while the defense was given the runaround by Athletic's aggressive forward line.
Gerard Pique was poorly positioned for the goal and is increasingly culpable for Barca, while his partner Javier Mascherano is enduring a torrid time this season. And in the full back positions, Martin Montoya and Adriano (playing in the absence of injured duo Dani Alves and Jordi Alba) looked out of their depth. With every passing week, the decision to dispense with Eric Abidal appears more and more ludicrous.
Captain Carles Puyol is beyond his best and is not the answer, either, while Marc Bartra is blossoming into an excellent center back, but is not at his happiest against physical opponents like Athletic. As has been the case for the last two years, Barca badly needs to sign a specialist center back. So why not in January?
The absence of first-choice goalkeeper Victor Valdes remains a problem, too, and the Catalans must ensure a replacement is brought in sooner rather than later because the Spain shot-stopper has revealed he is leaving in the summer.
Deals for Marc-Andre Ter Stegen and Pepe Reina are already at an advanced stage, but a new goalkeeper is only the tip of the iceberg for Barca. With problems appearing all over the pitch, Martino must dip into the market in January, because Barca is now level on points with Atletico and only three clear of Real Madrid in La Liga, while it will face far stronger sides than Ajax in Europe at the end of the season.
"We lost because they scored a goal and we didn't score any," sporting director Andoni Zubizarreta said after the game, but such analysis will get Barca nowhere. And nor will blaming the referee. Instead, the league leaders must look at where they went wrong and, as the January window approaches, the time to act is now.
Follow Ben Hayward on