Jeremy Mathieu brought Barca level a minute into the second half but another mistake from Mascherano enabled Ibai Gomez to steal possession in the box and guide a shot past debutant goalkeeper Jasper Cillessen.
“We were fragile in defence, and are where we've been strong in other years, but we also lacked fluidity and precision," he said.
“We were lacking inspiration. We didn't position ourselves well and we didn't occupy the right spaces in the first half."
He added: “The second Alaves goal did a lot of damage because it was a demonstration of our defensive fragility.
“It came from our own defensive error."