Barca were beaten by Kevin-Prince Boateng and Sulley Ali Muntari goals.
"We were lacking freshness in terms of ideas. They were well positioned on the field," Iniesta told UEFA.
"There is a second leg to play and we have to believe that we can come back from this. It will be a different story at our own stadium.
"From the very first minute at the Camp Nou, we'll be going for it."