The 18-year-old Brazilian only joined the Serie A giants in the summer from Corinthians, but he has emerged a first-team regular.
Now considered one of the best young defenders in Italy, City have been linked with a possible summer swoop - but Mancini does not think Roma will sell.
Asked if he was interested in the Brazilian by La Gazzetta dello Sport, Mancini responded: "Marquinhos I like a lot, but I do not think that Rome is willing to sell it.
"With just two signings next year the Giallorossi can win the Scudetto."
Mancini was again asked about a possible interest in Liverpool striker Luis Suarez - who he likened to his former player Mario Balotelli.
"Suarez reminds me of Balotelli. Partly because he looks for trouble, but there is also aggressiveness towards him," he said.
"As player, however, we have not discussed this."