Bilbao stunned Barca at San Mames 1-0.
"We had the game under control, but the goal really hurt us. San Mamés is always a tough place to come to," said Iniesta.
"Defeats always happen for a reason, but I don't think we can compare this one with the game against Ajax. We had some good phases of play tonight, but we have to be more efficient.
"We put in a good first half and we played well at times after the break, but the goal really shook us and although we really tried, we couldn't come back."
For breaking stories and all the great banter like us on Facebook: facebook.com/tribalfootball