Hart, who was also below his best at times last season, was at fault for two of Bayern Munich's goals in the Bavarians' emphatic 3-1 defeat of City in the Champions League on Wednesday night.
Nasri and Richards, however, have backed Hart to silence his critics, with Nasri regarding the England goalkeeper as one of the top five glovemen in the world.
"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," French playmaker Nasri is quoted as saying in The Independent.
"(Iker) Casillas doesn't play any more 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."
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."