Sergio Busquets took responsibility for the mistake which cost Barcelona a 1-0 defeat against Rayo Vallecano on Wednesday.
The Barca midfielder lost the ball in the build-up to Radamel Falcao's 30th minute strike as Barça lost for the second time in a week following Sunday's Clasico defeat.
"I made a mistake which was decisive because it led to a goal," Busquets said. "I was sleeping, they picked my pocket and it was completely my mistake."
Barca could have equalised in the second half, missing several chances including a penalty which was won and taken by Memphis Depay.
On another setback, Busquets added: "It was a little bit of the dynamic we have been going through all season. We're lacking a bit of everything. Luck, belief, concentration, a lot of things. In general, we're not playing bad games, but adding everything up it's very difficult.
"We had a lot of chances, Sergiño (Dest), Gavi, Aguero, even the penalty. It's worrying because they're not disastrous performances. The details that we're lacking are not a serious problem, the serious thing is that we're not getting points and that's conditioning things."
Asked if coach Ronald Koeman was to blame, the Barça captain said: "We're all in the same boat. We're all to blame. For good and for bad we're a team and we have to show that in victory and defeat."