Tuesday's 3-0 win over Liverpool at the Etihad Stadium re-established a three-point gap over neighbours Manchester United.
That could be closed again if Sir Alex Ferguson's side win at Newcastle tonight, but Mancini reckons the next month is vital to his own team's chances.
City will lose Yaya Toure, scorer of the second goal last night which capped an impressive performance, and his elder brother Kolo to the African Nations Cup after this weekend's FA Cup tie against United.
The former's absence will leave a big hole in their midfield but Mancini believes if they can overcome that and maintain top spot they will be in pole position in the race for the Premier League crown.
"This month without Yaya or Kolo when we play every three days that will be hard," said the Italian.
"January is a very crucial month for us because if we can stay on the top after January we have a big chance to win (the league)."
Mancini reckons there is now a three-way race for the title.
"I think United are a strong team but we should think only of trying to win every game if possible and at the end we will see what happens," he added.
"Tottenham are also very close but I hope we stay there (at the top of the table) for a long time."
City enjoyed an easier-than-expected win as Jose Reina's error in allowing Sergio Aguero's shot under his body gave them an early boost before Yaya Toure headed in David Silva's corner.
Even though they had Gareth Barry sent off for a second yellow card with 18 minutes remaining, City hit their visitors on the counter-attack and, when Toure was brought down by Martin Skrtel, James Milner dispatched the penalty.
"This is massive for us. We were incredibly efficient and it's a great win," City captain Vincent Kompany told Sky Sports 1.
"It is one of the top six teams out of the way and now we know United and the other teams have to play each other."
Liverpool manager Kenny Dalglish was unhappy with elements of the performance but did not think his side deserved to be 3-0 losers.
"I am not so sure the scoreline reflects the way the game went," said the Scot.
"I am not saying we deserved to win but the lesson we learned tonight was if we are not clinical we are going to suffer.
"There are a lot of things that happened tonight which we can correct by ourselves and we have to go and do that.
"We had a great opportunity to open the scoring (through Stewart Downing). We conceded the first goal after we lost possession. The second was a corner which we'd had a warning seconds before it and the third was when they were down to 10 men and Toure has run the length of the field and won a penalty.
"It is a harsh lesson but we will learn from it."