Defeat to Southampton on Saturday - their third of the Premier League campaign - was the worst performance of the season to date.
Sky TV point to the five matches which have proved pivotal in ruining City's title race.
1-1 draw (h) v Arsenal - October 23, 2012
No disgrace in a point against Arsenal but City led through Joleon Lescott's first-half goal before being pegged back by a Laurent Koscielny equaliser with eight minutes remaining. United had won at Liverpool earlier in the day.
3-2 defeat (h) v Manchester United - December 9, 2012
Doing the league double over United last season no doubt played a huge part in earning City their first title since 1968 but United forward Wayne Rooney struck twice in the first half to put his side on top at the Etihad Stadium. Mancini's side appeared to have clawed back a hard-fought point as Yaya Toure and Pablo Zabaleta levelled matters before Robin van Persie, a City target in the summer, scored a deflected free-kick in injury time.
1-0 defeat (a) v Sunderland - December 26, 2012
A Boxing Day defeat at Sunderland meant City slipped further behind United heading into the new year. Adam Johnson, who was sold by Mancini after being deemed surplus to requirements, bent in the only goal of the game by catching City goalkeeper Joe Hart off-guard.
0-0 draw (a) v QPR - January 29, 2013
With striker Mario Balotelli all set to secure a move to AC Milan, City travelled to struggling QPR looking to keep pace with United at the top. Rangers defended for their lives and City could not break through their resolute back line, having to settle for a single point against the league's bottom club.
3-1 defeat (a) v Southampton - February 9, 2013
Mancini labelled the City performance "worse than poor" as they were outplayed for almost the entire 90 minutes by Mauricio Pochettino's marauding Southampton side. A handling error from Hart allowed Saints to take a 2-0 lead and an inexplicable own goal from Gareth Barry early in the second half gave Southampton a well-deserved win and leaves City languishing behind United as they look to defend their crown.