Manchester City's hopes of defending the Premier League title were dealt a major blow on a disastrous evening for the champions as Southampton secured a 3-1 victory at St Mary's on Saturday.
After Jason Puncheon opened the scoring for Mauricio Pochettino's team, an embarrassing mistake from City goalkeeper Joe Hart allowed Steven Davis to double Southampton's lead.
Edin Dzeko got one back for City but a calamitous own goal from Gareth Barry shattered Mancini's men, who trail leaders Manchester United by nine points after their third successive league game without a win.
United can now move 12 points clear at the top if they beat Everton on Sunday.
For Southampton, the victory means they are now six points clear of the relegation zone and, on this evidence, look destined to secure their Premier League status for another season.
After returning from African Cup of Nations duty on Tuesday, City's Yaya Toure started his first game since the 3-0 win over Stoke on New Year's Day.
Pochettino selected an attacking line-up featuring Rickie Lambert and Jay Rodriguez and the Argentine could not have asked for a better start.
Puncheon stole the ball from Barry in the seventh minute and fed Jay Rodriguez, who saw his effort smothered by Hart.
Unfortunately for the England goalkeeper, the ball fell directly into the path of Puncheon and he slotted home from the rebound.
Things went from bad to worse for Hart midway through the half as Lambert cut in from the right flank and hit his shot straight at the goalkeeper, but the ball slipped out of his grasp and was stabbed in by the onrushing Davis.
A flashing volley from the lively Rodriguez that missed the post by inches almost extended Southampton's lead just after the half-hour mark before Pablo Zabaleta saw his curling effort just drift wide.
Less than five minutes later, the City right-back was instrumental in the counter-attack that saw City pull one back, with Dzeko converting a close-range finish.
Southampton have the unwanted record of throwing away more points from winning positions than any other Premier League team, but they should have been given the chance to restore their two-goal cushion on the stroke of half-time when Toure seemed to bring down Rodriguez, only for referee Martin Atkinson to wave their appeals away.
If City thought their luck had changed, they were sadly mistaken.
Puncheon came close to grabbing his second of the game from Lambert's cutback two minutes after the break.
But Southampton didn't have to wait long for their next goal. Within 60 seconds, Lambert again found space down the left but his ball across the face of the box appeared harmless until Barry inexplicably sidefooted past a helpless Hart.
Mancini threw on James Milner for Samir Nasri in an attempt to chase the game but once more it was the hosts who looked more likely to extend their lead.
With City still struggling to create any real chances, Maicon replaced David Silva and finally they looked a threat going forward.
Saints goalkeeper Artur Boruc did well to hold on to the ball following a goalmouth scramble, but Mancini cut a frustrated figure down on the touchline as his side continued to struggle for inspiration.
Some City fans tried to put on a brave face on things as they sang a tribute to Mancini in the dying moments, but this insipid display could end up being even more significant for the Italian's future than it is for their title prospects.