City headed into Wednesday's second leg of the last 16 tie at Camp Nou with a daunting 2-0 deficit to overturn, and were beaten 2-1 despite a much-improved display from their under-par showing at the Etihad Stadium.
The game was marred by a number of questionable decisions, with Barca not being awarded a first-half penalty when Joleon Lescott brought down Lionel Messi in the area before a Neymar strike was incorrectly disallowed for offside.
It was that decision that led to defender Zabaleta's dismissal, the Argentinian seeing red for a second bookable offence after protesting vehemently to the official.
"I was on the pitch but it was very clear - Pique came from behind and made the tackle on Edin Dzeko, and the referee was only two metres away. He saw that very clearly," he said.
"It was also a foul against Dzeko just before Barcelona's first goal. He held the ball and turned, and Pique had hold of his shirt, and the ref gave a free-kick to Barcelona.
"Sometimes in the game you are nervous because you are trying to get the result, and sometimes shocking decisions by the ref make you a bit angry.
"I don't know why I got a red card because I went to him to talk in a good way. I was a little bit angry to him because he didn't give the penalty but I was always respectful to him.
"I am very frustrated at the decision because it was a shocking decision.
"I am disappointed but also very proud of the team because I think we played a fantastic game.
"We tried from the beginning until the end, should have scored first and had more chances in the game."
