The new boys certainly proved their giant killing act once again, having beaten Real Madrid earlier in the season as well with Barca brought down to earth from their midweek exploits in the UEFA Champions League.
Barca now trail the league leaders Atletico Madrid by 12 points from seventh spot, which Koeman concedes is a lot to grasp.
"It's a giant step backwards in our chances of fighting for LaLiga, It's disappointing, but we have to accept it. A gap of 12 points to a team as strong as Atletico Madrid is a lot," Koeman told Marca.
This defeat also brought to light the team's form away from home in the league, which once again reared its ugly head much to the Dutch legend's frustrations.
"There are still a lot of games to play but if we don't improve our attitude and our ups and downs away from home, we can't say any more.
"We lost due to an unexpected mistake that you cannot make. The attitude wasn't good tonight, and not just from the defenders," Koeman said.
"It's very hard to explain some of the goals that we've conceded. I think it might be because of a lack of concentration. We lack aggression without the ball.
"That might be why we concede. It's not easy to explain that second goal," Koeman added.