Luis Suarez said it felt like it was "two points dropped" after China striker Wu Lei denied Barcelona all three points as Espanyol claimed a 2-2 draw in a pulsating city derby.
Wu struck two minutes from time after Barcelona were reduced to 10 men when Frenkie de Jong was sent off.
The Spanish champions looked on course to extend their lead at the top of La Liga when Arturo Vidal put them 2-1 up after Luis Suarez cancelled out David Lopez's opener for Espanyol.
The tide turned on a gripping match at RCDE Stadium, however, when Dutch midfielder De Jong was dismissed for a second booking 15 minutes from time and Wu took advantage of Espanyol's extra man to earn La Liga's bottom club a precious point with a powerful finish after 88 minutes.
Barca still top the standings on goal difference after 19 games but Real Madrid's 3-0 win at Getafe earlier on Saturday enabled Zinedine Zidane's side to move level on points with 40.
"We did the hardest thing which is to come from behind away from home and take the lead, so it feels like two points dropped," Suarez said.
"It's always tougher with one player less as the opponent starts to grow in confidence, but it's not an excuse. We are the champions and we are obliged to win every game so we need to come up with some solutions."
Ernesto Valverde's side went into their first match of the new year at the top of the table, while Espanyol were bottom and hoping for a new lease of life in their first game under Abelardo Fernandez, their third coach of the season.
Espanyol have not beaten their glamorous neighbours in the league since 2009 but they made an excellent start and deservedly went ahead in the 23rd minute when defender Lopez escaped his marker to nod home Marc Roca's free kick.
Barca looked flat throughout an uninspired first half with talisman Lionel Messi's headed effort on the stroke of half-time their only notable chance.
Valverde replaced Ivan Rakitic with Vidal for the second half and Barca were soon level as Suarez prodded in Jordi Alba's cross.
Suarez then turned provided for Chile international Vidal to head home to put the visitors in front.
De Jong received his second caution of the match after committing a needless foul and from then on Barca found themselves on the back foot. Espanyol's Wu came close to equalising when his header was saved by Barca's stand-in goalkeeper Neto, but the Chinese forward soon got the better of the Brazilian to give his coach Abelardo a memorable debut against his former side.
Valverde said the red card was the turning point of the match.
"Everything was in our favour, then we were knocked out of shape," he said. "We feel bad because we wanted all three points."