Quique Setien has said Barcelona's players still don't totally understand how he wants them to play after Valencia inflicted his first defeat as the club's coach since replacing Ernesto Valverde earlier this month.
The loss could see Barca, who beat Granada and Ibiza in Setien's first two games in charge, replaced by Real Madrid at the top of the table on Sunday, when Zinedine Zidane's side travel to Real Valladolid.
"We were not good at all, especially in the first half," Setien said in his news conference. "We weren't able to find a way through to goal. There were things we didn't understand properly and Valencia took advantage of our mistakes.
"The [players] still aren't interpreting what we want to do -- or maybe we're not explaining it well. Positionally there are things to correct and improve. No one likes what we have seen today."
Barca had over 70 percent of the possession at Mestalla but struggled to create anything in the first half. Valencia, meanwhile, won a penalty with their first attack in the 10th minute.
Marc-Andre ter Stegen saved Gomez's spot-kick and the German made a number of other stops to ensure the game remained goalless at halftime, while Barca's passing failed to yield to a significant opening in the first 45 minutes.
"We made so many passes for no reason, with no variety," Setien added. "That's not what we want. We were not good in the first half. The best thing was getting to half time at 0-0.
"We improved a little bit in the second half and we could have got back in the game, but that's not good enough for us."
Alba's own goal opened the scoring in the 48th minute and Uruguayan international Gomez sealed the points in the 77th. In the middle, Lionel Messi came close a couple of times, and Setien said he was happy with the way his players fought.
"It's not an attitude problem," he said. "The players looked committed. We just weren't well-positioned, we haven't understood certain things. We struggled. We will analyse the game and look for solutions."
Barca now have back-to-back home games in the next seven days, starting with Leganes in the Copa del Rey and followed by Levante in La Liga.