The France international says the Catalans were not causing too many problems before a penalty was awarded against Martin Demichelis and insists Bayern Munich is the better team.
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Nasri is drawing inspiration from Valencia's 3-2 triumph at Camp Nou earlier in February as City looks to knock Barcelona out of the tournament and reach the quarterfinals.
"[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 opponent of the season so far - an accolade he reserves for European champion Bayern Munich, which 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."