The result left Sociedad in sixth spot in the table, but they are just one point ahead of seventh-placed Villarreal with nine games remaining.
"We weren't good at all defensively and though we did a lot of good things in attack, we lost because we weren't good enough," Inside Spanish Football quotes Arrasate as saying.
"We've been working hard to ensure we don't repeat that.
"It was a real blow for us, but I'm certain we're beyond that now and we're already thinking about Thursday's game, because it's very important."
Sociedad face Real Valladolid on Thursday evening.