Balotelli was selected ahead of Carlos Tevez to start up front in the 3-2 defeat to Manchester United.
But he produced an anonymous display and was replaced by the Argentinian early in the second half.
Those remarks in turn fed speculation over Balotelli's future, with reports the former Inter Milan frontman could return to the San Siro in a loan move to AC Milan.
City have also been linked with various strikers – including Atletico Madrid's Falcao – in the January window.
But Mancini said: "Balotelli has enormous quality, he is not for sale. He is a player of 22 years old who has the quality to change a game at any time. I hope this moment will come soon."