The Italian started in place of Carlos Tevez in Sunday's derby against Manchester United but was hooked for the Argentine after 51 minutes because of a poor performance in the 3-2 home loss and Mancini has urged him to produce more.
"I love Mario but it is important for him to start to think about his job," said Mancini.
"He has everything to play well but he can't continue to play like today. We wanted more from him.
"When you have a player that has Mario's quality, you cannot understand why he continues to throw it out of the window. It is incredible.
"I have seen players in my life with fantastic quality. But in the end, they did nothing.
"I don't want Mario to finish like these players. It would be bad for him."