Deportivo La Coruna shocked Barcelona 2-1 on Sunday.
Joselu, on loan from Premier League side Stoke, opened the scoring for the hosts with just five minutes remaining of the first half at the Riazor.
Luis Suarez drew the defending champions level moments after the interval with his 21st league strike of the campaign, but Alex Bergantinos headed home Depor's winner with 17 minutes to go.
"The final third is always the most difficult," said Barca coach Luis Enrique. "There's not as much time to think. We lacked a bit of clarity in that area and it's a shame because this was an important game to continue in the same line and to show our rivals how convincing we are."
He added: "We knew it would be a struggle. From the beginning you could see that. We started well but we lacked a spark. Depor grew into the game and we began to make mistakes, which they took advantage of.
"The first goal was the key, or at least that's what I felt. Despite equalising they were still breaking through. We lacked clarity and that was a shame. We have to accept the defeat and look to the future, there are still a lot of games to go."