The Catalans fell behind early and after drawing back level in the second half, a lapse of concentration saw them punished, with Alvaro Negredo restoring the hosts' advantage just six minutes later.
"The defeat is difficult to explain," Koeman said after the game. "We've come here after some good games, but the first half wasn't good.
"We were better in the second half, but the way in which we lost was incredible. We lost due to an unexpected mistake that you cannot make. It came from not being focused. The attitude wasn't good tonight, and not just from the defenders.
"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."
Barcelona had appeared to have turned things around lately, and many had even suggested they might be able to get back into the LaLiga Santander title race, but this latest setback might change the narrative around the club.
"It's a giant step backwards in our chances of fighting for LaLiga [Santander]," Koeman acknowledged. "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.
"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."