The Spanish champions were beaten 3-0 in Turin by Massimiliano Allegri's side, with Paulo Dybala scoring twice before Giorgio Chiellini's second-half header.
Barca enjoyed more of the possession but struggled to place Gianluigi Buffon's goal under any real threat as they slumped to a second significant away defeat in a row in the competition.
Although they made history by overturning a 4-0 loss to Paris Saint-Germain in the last round with a 6-1 second-leg victory, Iniesta says any similar comeback will be beyond them if they do not sharpen up at Camp Nou.
"We did some bad things, mainly in the first half," he told TV3. "The Champions League punishes you with these two goals.
"In the second half we saw something different. We deserved to score one goal but that doesn't help now.
"Juve are a good team but when you don't do things well, you are penalised a lot.
"The feeling is not as pessimistic as it was in Paris but once again we have a real uphill task in a knockout game.
"If we do things as we have to, we'll be back in the tie. If not, it will be impossible."