The England No 1 was at fault for two Bayern Munich goals as the Champions League holders cruised to a 3-1 victory over City at the Etihad Stadium on Wednesday.
The 26-year-old has been under intense scrutiny for almost a year after a series of high-profile errors, but Nasri insists Hart remains among the best goalkeepers in the world.
The Frenchman said: "I think it's (Manuel) Neuer, Joe Hart and maybe Hugo Lloris, because I play with him in the national team, then (Gianluigi) Buffon too.
"(Iker) Casillas doesn't play anymore but for me 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."
Hart got a hand to Franck Ribery's shot but failed to keep it out as Bayern took the lead, and he was beaten at his near post by Arjen Robben for the German side's third.
But Nasri added: "I think that when you are a top player like Joe, you don't have problems with confidence.
"Sometimes you can have a rough patch but he knows everyone is behind him at the club."
Richards added: "Me and Joe are very close and we all make mistakes. He will bounce back because he is a strong character.
"He is not to blame for the result, the team were not good enough in all areas. He will bounce back.
"He is still England's best keeper. I know we have some real good talents but he is definitely the best.
"He knows he could have done better with the first, but he will get over it."