Mancini's title hopefuls travel to Sunderland on New Year's Day before hosting Liverpool on January 3.
City then face arch-rivals Manchester United in the FA Cup and have a two-legged Carling Cup semi-final with Liverpool as well as games against Wigan and Tottenham before the end of the month.
Mancini said: "It is impossible to play in two days with the same players, this is sure.
"January will be a key month. We will have important games - FA Cup, Carling Cup, four in the Barclays Premier League.
"I think this month will be very important."
City saw their two-point lead at the top of the Barclays Premier League wiped out as they were held to a goalless draw at West Brom on Boxing Day.
They now lead the table only on goal difference from Manchester United and could even be second by the time they kick off at the Stadium of Light as Sir Alex Ferguson's men return to action on Saturday.
Mancini anticipates a tight battle with the neighbours throughout the winter and spring and believes the final reckoning could all come down to their derby clash in April.
United travel to the Etihad Stadium for their third to last game on April 28.
Mancini said: "If we are strong enough, I think it will be decided with the game at home, probably."
Mancini expects right-back Micah Richards to be fit for Sunderland after being rested at West Brom due to a calf problem.
Left-back Aleksandar Kolarov is also coming back into contention after six games out with a groin injury.
Mancini said: "We didn't want to take any risk with Micah because we play a lot of games in the first 10 days of January. It is better.
"Kolarov is okay - it is the same situation."