Goals from Lionel Messi and Dani Alves at the Etihad Stadium on Tuesday leaves Manuel Pellegrini & Co. with an uphill task ahead of the return clash on March 12.
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"[Valencia] played without fear. That is what we need to do. We have to gamble, try to play and attack them. They do not like to be attacked," he told reporters.
"When it is a big game like this the two teams are there like boxers in the first round, trying to see how your opponent is.
"After that, we started to press them high and we saw they were like any team. They lose the ball. That is what we need to do. We will have to gamble? We went to [Bayern] Munich and beat them [in the group stage].
"They did not have any clear chances, but it is not over. We can do it. It will be an open game, they will come like Barcelona. We can do the same. Everything is possible. 3-1? We can do it."
Despite the first-leg loss, Nasri does not consider Barcelona as City's toughest opponents of the season so far - an accolade he reserves for European champions Bayern Munich, who won 3-1 at the Etihad during the group stage.
"Bayern are the best team we played here," he added. "Against Bayern, for 60 minutes we did not touch the ball. They are the best team, Barcelona are not. If it was not for the penalty, it was 0-0. Bayern was not the same.
"We did not touch the ball, they scored goals without a mistake from anyone. They showed us how great they were. I am convinced that Bayern are the best team we have played against."