Quique Setien suffered his first defeat as Barcelona boss as Valencia secured a 2-0 victory at Mestalla to preserve their unbeaten home record in LaLiga this season.
Marc-Andre Ter Stegen produced a string of top saves to keep Barcelona in the game in the first half, denying Maxi Gomez from the penalty spot after Gerard Pique had brought down Jose Gaya in the area, before tipping another Gomez shot onto the bar.
But Valencia took the game away from them after the break, drawing first blood three minutes into the second half thanks to a Jordi Alba own goal, before Gomez made the points safe 13 minutes before the end.
Barcelona remain top of the table but could be overtaken when second-placed Real Madrid take on Valladolid on Sunday.
Quique Setien conceded that he may not have explained things properly to his players.
In a post-match interview shown on Barcelona's official website, Setien said: "I think in the first half we were frankly poor.
"We weren't positioned well, we didn't have that precision, that ability to wait. They had five or six chances on the counter-attack.
"We have to evaluate a lot of things from the first half. I don't think we played well at all in the first half. There are things that we didn't really understand - perhaps I didn't explain them properly to the players.
"I think perhaps we need to understand things in a bit of a better way.
"It's true that in the second half we were able to have more space and get forward more. We had four or five chances. Some of them were really clear and we could have got into the game that way.
"But then unfortunately we weren't able to get the goal and then they scored the second on the counter and then it was really difficult."
Goalscorer Maxi Gomez told the club's official website: "I am very happy to help the team, we deserved this victory.
"Now we have to continue. Go game-by-game and hopefully continue with this streak."
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