The Italian had been strongly linked with a move to England but, as Goal.com exclusively revealed on Monday, he has decided to remain with the club that he joined at the age of 17.
City had prepared an offer of £31 million for the Italy international and were willing to hand the midfielder a lucrative contract which would have seen him paid £7m a year. However, though Roma were receptive to the possibility of allowing the 29-year-old to leave, De Rossi announced his decision to reject a move at a press conference on Tuesday.
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"I can tell you the truth. I came to do a classic press conference before the season. I wanted to make a clarification.
"What I can promise everyone is that the day I want to leave Roma - whether to win the Champions League or make more money - I will say so in public.
"This is a normal press conference for me, not the announcement of the Third World War. For me it's important to say my piece. I am here at Roma, I'm happy and never asked to leave.
"I don't know if club still want to sell me. Guessing they count on me since they gave me huge contract in February. Ask them."
The news comes as a blow to Mancini, who lost out to Chelsea in the race to sign Eden Hazard and similarly failed to capture the signature of Robin van Persie prior to his move to Manchester United.
The club have made just one signing thus far this summer, bringing in young midfielder Jack Rodwell from Everton in a deal believed to be worth £15m.
De Rossi has 79 Italy caps and has won the World Cup with his national side, but has failed to match that success with Roma - who have finished as Serie A runners up on five occasions during his stay.