A late Cristian Tello goal saw Madrid follow its El Clasico win with a loss at the Benito Villamarin, a defeat that midfielder Casemiro admitted "could cost us the league" in the latest twist in a rollercoaster title race, ESPN reported.
With just 11 games left to play, Barcelona now leads the La Liga table on 58 points, with Real Madrid two points behind it. Madrid has lost three out of its last four games in all competitions.
"I think we played a bad game, from beginning to end," Zidane said in his post-match news conference.
"It's a shame, because we had the chance to play a good game and we didn't. We didn't play at all. I don't have an explanation."
"I'm the coach and when things don't go well, or we don't do what we'd talked about, I'm responsible," he added. "It was our worst game of the season. Defensively, offensively, our energy, we didn't have enough mobility to cause the opponent problems. We conceded two goals, but what annoys me most is what we did with the ball."
Real Madrid's defensive record has impressed this season, conceding the fewest goals of any team in La Liga, but two mistakes in Seville – from Sergio Ramos and Karim Benzema respectively – were costly, with each leading to a Betis goal.
"The result is fair, we have to be self-critical, we weren't good today," captain Ramos said after the match.
"It's not enough, we should have had more intensity and fought more. There are 11 games left and a lot of points at stake. If we want to be champions, we can't pass up opportunities like that."
Midfielder Casemiro went even further, admitting that the loss could have dire consequences for Madrid's title hopes.