Barcelona coach Gerardo Martino and captain Carles Puyol have demanded more intensity from the players after Tuesday's 2-1 Champions League defeat at Ajax Amsterdam.
The Group H reverse was Barca's first loss this season in all competitions - and their first under Martino - which cost them a chance to seal the top spot, having already secured their place in the last 16.
A fired-up Ajax, roared on by their vocal fans, took the game to an injury-hit Barca in the first half, harrying and hassling them before taking a deserved 2-0 lead with goals from Thulani Serero and Danny Hoesen.
Although the Spanish champions dominated after Ajax were reduced to 10 men shortly after halftime and pulled a goal back through Xavi's penalty, they struggled against a home defence well drilled by coach and former Barca player Frank de Boer.
"Ajax began with more intensity than us and we paid the price," Martino, who took over from the ailing Tito Vilanova in the close season, said at a news conference.
"The second half was a different story and we played in a way more in tune with what Barca represents," the Argentine added.
"The problems in the first half came more when we had the ball than when we didn't.
"We lacked mobility and precision. In a Champions League match against a rival who is fighting to qualify for the next round you need intensity for the full 90 minutes."
Barca have another chance to seal the top spot in the group when they host Celtic on Dec. 11. The Scottish champions lost 3-0 at home to AC Milan on Tuesday and are bottom of the group on three points from five games.
Barca have 10, Milan are second on eight and Ajax, whose final game is against the Italians at the San Siro, seven.
Puyol said the team would have no chance of adding to their four European crowns if they started every match so lethargically.
"The idea is to have possession of the ball but when you are up against a team that also knows how to play and is full of fight this is what happens," he told reporters.
"If you do not have enough intensity they will roll right over you.
"You have to gain experience from all your matches and if we want to win titles we have to improve."
Puyol's defensive partner Gerard Pique said it was Barca's worst first-half performance of the season.
"We played very badly and we were lacking in commitment," added the Spain international.
"In the second half, although we improved, we did not create enough chances to get the second goal.
"They can score a goal or dominate during certain periods but what happened in the first half cannot be allowed to happen. That is not the image of FC Barcelona."
Barca's next match is at La Liga rivals Athletic Bilbao on Sunday, when they will be seeking a 14th win in 15 matches but will again be without World Player of the Year Lionel Messi and goalkeeper Victor Valdes, who are both injured.
The Spanish champions, unbeaten in La Liga, have a three-point lead over second-placed Atletico Madrid. The only points they have dropped were in a 0-0 draw at Osasuna last month.
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