A shambolic display by the champions could leave them 12 points adrift of Manchester United by the end of the weekend.
Three defensive howlers from Manchester City, each one of increasing and barely believable ineptitude, helped Southampton romp to a 3-1 win at a rocking St Mary’s Stadium.
Jason Puncheon and Steven Davis took advantage of errors from Gareth Barry and Joe Hart to put the Saints 2-0 up after 23 minutes.
Edin Dzeko gave City hope with his 12th league goal of the season shortly before halftime but Barry gave fresh meaning to the term ‘comedy own goal’ with a lethal side-foot into his own net two minutes into the second half.
Vincent Kompany travelled with the City party and was watching from the stands but he was still not fit to return to the 18-man squad.
How Mancini must have lamented the absence of his talismanic skipper, firstly when Southampton unlocked the re-jigged City defense in the seventh minute.
Jason Puncheon used his pace to wriggle clear on the right, threaded a neat ball through to Jay Rodriguez, whose shot was well parried by the alert Joe Hart. However, the loose ball fell to Puncheon, who tucked in his seventh goal of the season.
City struggled to gain a foothold in the match and a goalkeeping howler midway through the opening period gifted the hosts their second.
Hart’s dip in form has coincided with a national television advert in which he promotes a shampoo, and the entire City staff were left tearing their hair out when he allowed a Rickie Lambert shot headed for his midriff to squirm out of his grasp, between his legs and into the path of the grateful Steven Davis, who tucked it in.
City had been desperately poor for most of the first half but there was no doubt about the quality of the goal which signalled a comeback could be on.
After Dzeko had headed clear a Southampton corner, he raced up the other end to finish off a counterattack that had been beautifully worked until Pabo Zabaleta’s cross was squared behind the Bosnian. Dzeko slipped as he checked his run but somehow managed to get a foot on the ball to stab in his 12th league goal of the season.
Shortly after Puncheon had blazed over the bar, the home side gained further reward for its purpose and attacking intent following a brainfart from Barry.
The lively Lambert broke free down the left but his cross should have been comfortably dealt with by Barry, who, under no pressure at all, inexplicably passed the ball into the bottom corner of the net.
Most observers expected some sort of reaction from City, a late assault on the hosts’ goal or the kind of sustained comeback for which it has become renowned over the last two seasons.
But it never really materialized. Southampton drew the sting from the visitors with relative comfort and David Silva was mysteriously removed from the fray by Mancini with 17 minutes remaining.
With the win, Southampton now sits six points clear of the relegation zone. Meanwhile, City's title defense has almost completely disintegrated.