After waiting 44 years to claim the crown of English champions, Manchester City appear to have relinquished it within just 26 matches.
Sergio Agüero's triumphant conclusion to last season's campaign seemed an irrelevant relic here as City began sluggishly, briefly flickered and then floundered like dishevelled brawlers before being floored by slick Southampton. Manchester United can move 12 points clear of their neighbours with 12 games to go if they beat Everton on Sunday and even if they fail to open up such a gap, this does not look like a City squad that can mount a comeback on a par with last season's.
Mauricio Pochettino's Southampton, meanwhile, look nothing like a team at risk of being relegated. They have earned plaudits since the Argentinian replaced the unfortunate Nigel Adkins last month but this was the first win in four attempts under the new manager. It was amply deserved. They are now six points clear of 18th place and seemingly heading upwards.
Mancini's confidence of avoiding a slip up at St Mary's must have been increased by the return of the Ivorian Yaya Touré, who, it was hoped, would make Southampton suffer his anger at his country's failure to reach Sunday's Africa Cup of Nations final. City did not lose any of the five matches that Touré missed while he was away but draws in the last two meant they did lose ground. While Yaya took the captaincy because Vincent Kompany was still missing with calf trouble, his brother Kolo only made the bench. Javi García partnered Joleon Lescott in defence. Kompany's absence was to prove costly, as was that of Carlos Tevez, who was left out for what the manager termed "a personal problem" without elaborating.
Southampton had problems too, with Luke Shaw and Gastón Ramírez out injured. Danny Fox came in at left-back while Pochettino chose to deploy Steven Davis in midfield ahead of the fit-again Adam Lallana.
The Argentinian manager was hired in the hope that he will propel Saints to the upper echelons of the league but the cheering that greeted the announcement before kick-off that Wigan, Queens Park Rangers and Reading had all lost gave a reminder that the club's immediate concern is averting relegation. Their chances of doing that began to look brighter as soon as the game began.
From the outset Southampton fervently pressed City high up the pitch in the manner that Pochettino has demanded since his arrival. The visitors failed to match their intensity and were punished after just seven minutes, when Jason Puncheon robbed Gareth Barry near halfway and pinged a pass into Jay Rodriguez at the edge of the area.
García was caught badly out of position but Touré and Hart combined to keep the forward's effort out, only for the ball to break to Puncheon, who rolled it into the net from 13 yards.
Now City had to show the stuff of champions. They improved enough to force Southampton onto the back foot but not to create clear opportunities. The home team coiled up expertly and then, in the 22nd minute, sprang forward to deadly effect. Fox instigated the attack with a splendid crossfield ball from the back, which Rickie Lambert trapped before exchanging a one-two with Puncheon and unleashing an 18-yard shot that Joe Hart should have held. Instead the England goalkeeper dropped the ball and Davis slid in to poke the ball into the net.
City began to unravel. Rodriguez came close to making it 3-0 when he fired just wide in the 33rd minute, and five minutes later Puncheon blasted into the side-netting. In the 39th minute, City finally awoke, cutting the deficit thanks to a fine counterattack. From a Southampton corner they broke downfield before the ever-reliable Pablo Zabaleta crossed for Edin Dzeko to divert it into the net from close range. Artur Boruc had to intervene to prevent David Silva from equalising before the break but Southampton could have been given a penalty in the 45th minute when Touré leaned into the back of Rodriguez to send the forward tumbling. The game promised more goals but, bad as City were, few people would have foreseen their incompetence sinking to the depths that they reached in the 48th minute, when Lambert was allowed to canter down the right and send in a low cross that, bizarrely, Barry sidefooted into his own goal.
Barry attempted to redeem himself and clipped a canny ball through to Agüero in the 55th minute, but Boruc saved the striker's shot.
City needed goals and with Tevez missing and Mario Balotelli sold, Mancini had few ways to boost his attack from the bench. He replaced Lescott with Aleksandar Kolarov in the 66th minute and City switched to a back three. It made no difference. Southampton have surrendered more points from winning positions than any other team in the top flight this season but they held on quite comfortably here.