The 26-year-old conceded three goals in his side's midweek Champions League defeat to Bayern Munich, fumbling the opener from Franck Ribery and receiving criticism for the third from Arjen Robben.
However, Nasri believes that he remains one of the best goalkeepers in the world, backing him as England's No.1 amid speculation over his starting place for the Three Lions' World Cup qualifying fixtures.
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"Joe is in the world's top five, easily. I am not worried about him. For the first time in a long time, England have a world-class goalkeeper."
The France international expects his colleague to return to form in Saturday's early kick-off against Everton, where City look to bounce back from Premier League defeat to Aston Villa as well as their loss to Bayern.
"When you are a top player like Joe, you don't have problems with confidence," Nasri added. "Sometimes you can have a rough patch but he knows everyone is behind him at the club.
"The good thing for Joe and the rest of us is that we play [on Saturday] at 12.45BST so we don't need to think too long about what happened on Wednesday."