The loss in Pamplona means that the Catalans now have a mountain to climb in order to retain their Spanish title, with leaders Real Madrid now in a position to stretch their lead at the top of the table to 10 points with a win over Levante at the Santiago Bernabeu on Sunday evening.
Iniesta freely admits that he and his colleagues are hurting at the moment but knows that they can ill afford to wallow in self-pity if they are to successfully defend their European crown.
“It’s a painful loss," the Spain international confessed to his club's official website after the match. "We reacted [in the second half] but it wasn’t enough.
"We have to keep on fighting until the end [but] now we have to prepare for the Champions League match."
Iniesta's team-mate Isaac Cuenca insisted that Barcelona's belief in themselves had not been affected by the damaging defeat at the Reyno de Navarra.
“We’re going to Leverkusen with a lot of enthusiasm to try and win," the forward declared. "We know what kind of team we are and we must keep on battling."
After Tuesday's trip to Germany, Barcelona return to La Liga action against Valencia at Camp Nou next weekend.
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