Barcelona stumbled to a 3-2 home loss against Valencia on Saturday, giving Atletico Madrid a chance to move into sole possession of first place in the Spanish league.
The Spanish champion's decision not to sign a defender during the January transfer window may have backfired after Valencia took advantage of a lethargic, uninspired effort by its hosts at the Camp Nou, where former Spain coach Luis Aragones — who previously coached the Catalan side — was honoured after his death on Saturday.
Barcelona led through Alexis Sanchez's eighth-minute goal but Dani Parejo equalized for Valencia before the break.
Pablo Piatti headed Valencia ahead after the restart before Lionel Messi's penalty — and first league goal since September — levelled the game again.
But Barcelona's defence let Sofiane Feghouli burst free into the area to set up Paco Alcacer's 59th-minute winner.
Barcelona played with 10 men from the 78th after Jodi Alba was shown a second yellow card.
The defeat means Atletico can lead the league alone for the first time this season if it does not lose at home to Real Sociedad on Sunday, when Real Madrid can also overtake its greatest rival with a victory at fourth-place Athletic Bilbao.
Atletico and Barcelona share 54 points, while Madrid has 53. Valencia provisionally moved into eighth.
Barcelona coach Gerardo Martino was preoccupied by Barcelona's drop in play after just half an hour.
"I'm always worried after every one of our losses," Martino said. "Despite us having played well for a while against Valencia, I'm very worried, much like the players are."
Messi set up the opener after slashing a pass across the box beyond the far post where Alexis connected oddly to float it over goalkeeper Diego Alves.
Failed to build momentum
Barcelona failed to build on its early momentum. Valencia took control from the 44th when Parejo exploited a turnover inside the Barcelona half to lead a breakout with Feghouli, who returned a pass inside the area for Parejo to score easily.
"Their goal hurt us, and in the second half the team failed to respond," said defender Javier Mascherano, who refused to blame the off-field distractions that have enveloped Barcelona. "Today it wasn't the distractions that beat us, but Valencia. The margin of error (to be champion) is zero."
Valencia went ahead in the 48th when the 1.63-meter (5-foot-3) Piatti held off Dani Alves to jump high and head a looping ball over goalkeeper Victor Valdes.
Valencia was unlucky as Pedro Rodriguez redirected a shot off Ricardo Costa inside the area to set up the spot kick, even though it appeared Costa had no chance of getting out of the way as the ball grazed his shoulder.
Messi slotted home his ninth league goal in the 54th, and first since Sept. 28 against Almeria.
Barcelona's defence was dormant five minutes later as Feghouli moved around three players to the goal line before cutting back to meet Alcacer, who blasted past Valdes for the winner.
Messi had a golden opportunity to draw Barcelona even in the second minute of stoppage time, but he shot wide after precisely combining with Dani Alves and substitute Andres Iniesta to get free into the box.
Valencia, which was Spain's most active team in the January transfer window but used just one new recruit in striker Eduardo Vargas, snapped a five-game winless run to deal Barcelona its first home league defeat in nearly two years.
Also, Levante and Rayo Vallecano played to a scoreless draw, as did Getafe and Valladolid.
Later Saturday, Sevilla was at Malaga.