The champions ended January trailing their neighbours by seven points at the top of the table.
But in a month when United resume their Champions League challenge with a much-anticipated clash against Real Madrid, Mancini feels it could be time to strike.
United are at the Bernabeu on February 13, three days after a testing visit from Everton, and then host the Spanish giants on March 5.
Mancini said: "They are two important games for United so I think that February and March could be the time when it's possible to decide this championship."
City dropped two points as they were held to a goalless draw at QPR in midweek but Mancini does not see any reason to worry.
He said: "This doesn't change (anything). I think when we arrive at the end of February, the gap will be very, very small.
"In every championship, never one team won a title in January. We should fight until April.
"If they have seven, eight, nine points (lead) in April, it could be difficult, maybe. But in this moment we have a big chance.
"Every team during the season has two or three weeks when they don't play well, when their performances go down and they don't score.
"Even if you have fantastic strikers, like the situation we had last year, there were some moments when we scored three or four goals a game but then didn't score for three or four games and we didn't know why.
"It could be the same for them (United) now."
City face the challenge of Liverpool and their in-form striker Luis Suarez at the Etihad Stadium this weekend.
City were linked with a £40million swoop for Suarez, who has scored 22 goals this term, earlier in the season but Mancini did not want to get caught out on that subject at his pre-match press conference.
When asked about the Uruguayan, Mancini said: "Suarez? He plays for Liverpool. He has quality and is strong."
City will have to try to keep Suarez quiet without defensive linchpin and captain Vincent Kompany.
The Belgium international suffered another calf strain in last week's FA Cup fourth-round win at Stoke and could be out until after the fifth round against Leeds in a fortnight.
City have admitted concern over the recurring nature of Kompany's problem but he is not expected to need an operation.
Mancini said: "He has this problem. An injury to the calf is a bad injury.
"You can have this for a long time. Vinny needs to work and have treatment but I'm not worried about an operation.
"He did this problem in March last year after five minutes against Sporting Lisbon. We need to manage him well."