Xavi Hernandez admitted that Barcelona "didn't understand" what was needed to beat Shakhtar Donetsk in the aftermath of their Champions League group stage defeat.
The Catalan giants were beaten 1-0 by the Ukrainian champions in Hamburg, with Danylo Sikan's first-half header enough to earn three vital points in Group H.
Barcelona had just one shot on target during the game, with Shakhtar having four, and prolific goalscorer Robert Lewandowski has now gone six games without registering a shot on target - the first time the Pole has endured such a streak at club level since 2011.
Barca are still in pole position to qualify for the knockout stages, having won their three previous group stage games, but a furious Xavi admitted his side's level was not where it needed to be.
"We deserved to lose. We have to be honest. We played badly and nothing we had planned came off," he told reporters.
"The level of our performances is really low at the moment. This was one of the worst games of my two years in charge. We have to be self-critical. There was a reaction in the second half but it wasn't enough. This competition demands your best and we were not at it today. It is clear a step back.
"We are angry. Confidence has dropped, we are in a bit of a rut and we need a reset.
"We didn't understand what the game needed. We lost a lot of balls, we didn't attack well, we didn't deal with crosses well and we paid for our mistakes."
Xavi was similarly disgusted by his side's performance in a narrow win over Real Sociedad in La Liga at the weekend, labelling that showing as "completely unacceptable", and admitted his side need to overcome somewhat of a mental block.
"We have to restore the confidence. We're having a small dip right now. Not so long ago we were playing very good football - it's a mental issue."
Barca can secure their place in the round of 16 on November 28 when they welcome Porto - who are level on points with Xavi's side at the top of Group H - to the Lluis Companys Olympic Stadium.
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This article was originally published on 90min as Xavi reveals key Barcelona failing after dismal Shakhtar defeat.