City's title defence hit a new low after gifting all three points to Southampton. The Saints won their first match under new manager Mauricio Pochettino and a memorable one.
City midfielder Gareth Barry had a day to forget after his mistake led to the opening goal, Jason Puncheon slotted home after Jay Rodriguez's shot was saved. Steven Davis doubled the lead after Joe Hart's mistake in front of goal.
Nothing seemed to work for City as the Saints continued to pile up pressure by pressing high up the pitch. However, Edin Dzeko responded for City with a quick counter attack. Three minutes into the second half, Barry, under no pressure at all, makes the worst back pass to Hart which ends up in the bottom corner.
A devastating blow for City and their fans as league leaders Manchester United can extend their lead to 12 points with 12 games to go if they beat Everton at Old Trafford.
Prior to the game at St Mary's, City had the best defensive record in the league, conceding only 21 goals. But a horror show at the back, careless passing and an effortless attack ruined their title dreams.
Mancini admitted his side are missing first choice defenders captain Vincent Kompany and Matija Nastasic. With Javi Garcia and Joleon Kescott at the back, City were always under threat from Saints' pressing style of play.
City have not won in their last three league games, drawing against league strugglers QPR and came back to level 2-2 against Liverpool.
Southampton deserved to win against the reigning champions and moved six points clear off the relegation zone.
City suffered their heaviest defeat of the season and also their worst pass completion rate of 76%. The Sky Blues are slowly deteriorating as the season progresses and with games against Chelsea and United left, Mancini can only hope his side win all games and try to close the gap on United.