Defensive errors from the defending champions gift-wrapped a comfortable home victory for the relegation battlers, who have now only lost once since Christmas and appear under strong direction with new manager Mauricio Pochettino.
Jason Puncheon netted the Saints' first following a Gareth Barry mistake, who, almost inexplicably, would go one better later that game and pass the ball into his own net just after halftime.
Throw in an easy goal for Steven Davis after Joe Hart had made a meal of Rickie Lambert's, effort and this truly was an embarrassing performance for City.
They had not been beaten more than they had beaten themselves.
Coming to St. Mary's Stadium this week with the chance to pull points off Manchester United in the title race, City were largely expected to waltz through their opponents.
It wasn't that Southampton were particularly bad, it was just that, with everything to play for and the far better side on paper, City were going to win.
They were always going to win.
Right up until the moment they stepped on to that pitch and delivered arguably the worst performance seen from Roberto Mancini's men this season.
No wonder the manager was downright furious after the game.
Granted, Southampton played well and Pochettino's men deserve the respect they get after a big win over the defending Premier League champions.
But at the end of the day, this was City's game to win and City's chance to peg United back in the title race, and they walked away from Southampton with neither.
Should the Red Devils triumph over Everton at home this weekend, their lead in the Premier League title race will extend out to 12 points with just 12 games left.
Those 12 do include some challenging fixtures for United such as Arsenal, Chelsea and, of course, the return game against City at Old Trafford, but given their form this season and remarkable attacking prowess, it seems Sir Alex Ferguson's men with be very tough to catch now in the title race.
If United win this week, it could well be all over for City.
Which in reality, is very fitting and in line with the rest of their 2012/13 season—disappointing and nowhere near as good as what was promised.
After a remarkable year in 2012 that finished with Martin Tyler screaming "AGUERRROOOO" at the top of lungs, City game into the 2013 season with so much promise and so much expectation. They had strengthened their side in the summer and now, with the confidence that comes from wining the league title, would seemingly be an even tougher side to stop.
They had never started a season before with so much promise.
Now we know why.
They were bundled out in embarrassing fashion by lowly Aston Villa in the Capital One Cup, who would go on to lose to League Two side Bradford City. The Villains sit 19th in the Premier League at the time of writing this and weren't that much higher when they knocked out City.
Their European berth in the Champions League was even worse.
Drawn into the group of champions with the chance to pit themselves against the best in Europe, they didn't even qualify from the group stages with a win.
Alive in the FA Cup by the good fortunate of Stoke City's attacking incompetence, City looked to the 2013 Premier League title as the chance to resurrect what was an otherwise troublesome, problematic and spluttering season. And now they might not even be able to look to that.
Their attack still seems bereft outside of Sergio Aguero; their defense is leaking more than the red half of Manchester's was at the start of the season.
Roberto Mancini has reportedly been backed into almost odds-on to be the next manager sacked with the latest loss to Southampton—something that doesn't seem too far of a cry given the overall disappointing results that the Italian manager has achieved since arriving at the club.
One Premier League title and one FA Cup win are simply not enough. Not when you have the money and talent on hand that Manchester City have.
Mancini will seemingly be in a tough spot at the end of the season to fight for his place as the club manager, something that seems almost ridiculous to even contemplate given the success and glory that achieved for the Sky Blues last year.
Should Mancini stay?
And whilst one loss to Southampton doesn't suddenly change all of that, in the bigger scope of the 2012-13 season, Mancini is certainly under some real pressure to perform. Manchester City as a whole are under real pressure to perform and prove that they aren't the biggest one-hit wonders since Las Ketchup—something they didn't prove that this week against the Saints.
Lost in amongst the sheer elation of the Aguero Premier League-winning goal last year was the words, "I swear you'll never see anything like this again."
Perhaps he was talking to Manchester City all along.
We just didn't know it would come so soon.
Are Manchester City out of the title race with their latest EPL loss?
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