The Argentine dismissed Manuel Pellegrini's comments about the referee being biased towards the Catalan club and says he is feeling fitter with every game.
The Catalans defeated the Premier League side 2-0 on Tuesday, with the Argentine's penalty and Dani Alves downing 10-man City, who saw Martin Demichelis sent off after conceding a spot kick for tripping his compatriot.
As well as calling for calm, the Argentine also suggested Manuel Pellegrini's rant about the match officials being biased towards Gerardo Martino's men was just normal post-game complaining.
"It's usual that people talk about referees after the match, there are always things to comment on," the 26-year-old told reporters. "I did not talk with Martin Demichelis about the penalty against him. Little by little I'm feeling fitter.
"It's an important step to win at the Etihad Stadium. It is not definite that we'll qualify though, because they have a great team and great players."
Man City will visit Camp Nou in the second leg of the last-16 clash on March 12, with Barca back in La Liga action this weekend at Real Sociedad.