The Blaugrana suffered only their second loss of the La Liga campaign, with El Bilal Toure’s 24th minute goal the difference.
“It is the worst game of the season, both in attack and defence, as well as in construction and intensity,” Xavi told reporters.
“Nothing came off for us. We were very slow. Perhaps fatigue takes its toll on us. We have had a bad game.
“I’m angry because it was an opportunity to get to 10 points (ahead of Madrid) and we didn’t do it.
“We are upset and we have to find solutions now, because the decisive stage in La Liga and the Cup is coming and we cannot fail anymore.”
Barcelona remain comfortably on top of the table, holding a seven-point buffer from second-placed Real Madrid who drew 0-0 with 10-man Atletico Madrid on Saturday.
“It was a golden opportunity, Madrid having tied and we have had the worst game at the worst moment of the season,” Xavi added. “We have to change the chip and the mentality right now.
“Im very annoyed, it was a day to strike a blow, not a definitive one, but important one in La Liga.
“We have failed, we have no excuse, we have not shown intensity, desire to win, and that is what worries me the most. Then you can play better or worse.
“We can fail, but in the first half we didnt show passion or soul. We failed, we have to apologise to the fans. Today we couldnt play in any way, we lose a golden opportunity.”