Alvaro Negredo struck the winning goal after a series of surreal mistakes from Barca as Jordi Alba’s bungled throw-in flummoxed fellow defender Clement Lenglet and goalkeeper Marc-Andre ter Stegen kicked the ball against the forward, who gleefully followed it into the net, Reuters reported.
“We lost because of an incredible error, that is not the level I expect from my team,” Koeman told Catalan network TV3 after Barca’s fourth defeat in 10 league games.
“It shows we were not concentrating. The attitude was not good tonight. I’m very disappointed and we have to improve.”
Cadiz had taken an early lead thanks to poor defending from Barca following a corner, when the inexperienced Oscar Mingueza almost headed the ball into his own net but the host’s forward Alvaro Gimenez scored instead.
Barca equalized in the 57th minute through an own goal by Pedro Alcala only to fall behind again soon after.
The comedy of errors which led to Cadiz’s winner was the third consecutive away game in a row in which Barca has made unforced mistakes and failed to win.
“It’s very difficult to explain the goals we are conceding lately. It could be down to a lack of concentration,” Koeman said.
“When we equalized we had half an hour left to score again but then we conceded a goal that we cannot afford to concede, from a throw-in. We also lacked aggression when we didn’t have the ball today.
“We had highs and lows during the game and also in our attitude.”
Barca lost out on last season’s title to Real Madrid on the penultimate day of the campaign but now risks being knocked out of the title race with little more than a quarter of the season played.
“It’s a gigantic step back for our chances of fighting for the title,” Koeman added.
“We have to see it like that after getting such a disappointing result today. Twelve points is a very big distance but we have to keep going.”