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Sadio Mane's hat-trick helped Southampton thump Manchester City's disjointed and disinterested second string 4-2 in Sunday's Premier League clash at St Mary's.
Shane Long also found the net as Saints kept their Europa League qualification bid alive, strolling home against a City line-up distracted as much by fears for their individual futures as thoughts of the Champions League.
Kelechi Iheanacho struck twice for the visitors, but Manuel Pellegrini's largely reserve line-up wilted with precious little resistance on the south coast.
City boss Pellegrini made eight changes from Tuesday's goalless Champions League semi-final first leg draw with Real Madrid, gearing up for Wednesday's return at the Bernabeu.
The Chilean duly exposed the shortcomings in his squad building at the Etihad outfit however, with many on show rightly fretting that Pep Guardiola will wield the axe on arrival at the club this summer.
Pellegrini's rotation backfired spectacularly with City yet to close out a top-four finish to guarantee Champions League football next term.
But the former Real Madrid boss can justifiably lambaste the limp performances of a host of experienced hands, who could easily be shipped out by Guardiola before next season.
Fabian Delph, Fernandinho, Wilfried Bony, Pablo Zabaleta and Aleksandar Kolarov could all be expecting the worst on this evidence.
An entirely different City altogether will face Real in Madrid on Wednesday, and now Pellegrini must hope this resounding defeat carries no significance into Europe.
City's vast changes ripped out any rhythm, and after a meandering opening the hosts took full advantage.
Pablo Zabaleta had to scramble a recovery to deny Long, before Hart and Mangala were both forced to clear testing crosses from Tadic.
Steven Davis' neat through-ball set Long tearing into the inside-right channel, and his low first-time strike only just swept past Hart's near post.
Despite the warning shots City should have opened the scoring, Raheem Sterling's poor finish letting down Kelechi Iheanacho's persistence.
The Nigeria forward picked Virgil van Dijk's pocket on the touchline, drove into the box and cut back a smart ball to the onrushing Sterling.
The former Liverpool winger blasted his shot straight down the middle of the goal however, leaving Fraster Forster with a showy but expected save.
Then Saints changed the game, Tadic laying on two goals in three minutes to turn the tie.
First the Serbia playmaker latched onto Cuco Martina's angled ball and flicked a cross that Long slid through Hart's legs.
Then Tadic slid through a fine ball to send Mane clear, and he finished neatly past Hart.
The goals toppled an already-wobbling City. The visitors were overrun in midfield from the outset, spending the majority of the first half chasing shadows.
Mane, Davis and Tadic overwhelmed Fabian Delph and Fernandinho too often to credit the duo's berths in a squad still eyeing Champions League glory.
City wrestled their way back into the contest just before the break however, Martina diverting a poor clearance straight to Iheanacho.
The 19-year-old nodded home the gift, to claim City a foothold they hardly deserved.
Southampton quickly shrugged off Iheanacho's goal after the break however, with Mane pouncing twice more to complete a smart hat-trick and sink the woeful visitors.
First Mane slotted home on the rebound after Van Dijk's header from Davis' corner had crashed against the bar, then the 24-year-old slid the ball past Hart after another inch-perfect through-ball from Tadic.
Iheanacho struck again for the visitors with a fine finish after Jesus Navas' smart ball, but the damage was long since done by a group of nearly-men whose futures at City are most certainly in doubt.
TWEET OF THE MATCH
"All young defenders should watch video of Kolorov from the time Southampton win the ball 80 yards from City goal until Mane scores !!!" - Former City striker Rodney Marsh (@RodneyMarsh10) was less than impressed by Kolarov's part in Southampton's second goal.
Fraser Forster 7 (out of 10)
Cuco Martina 5
Jose Fonte 6
Virgil van Dijk 7
Ryan Bertrand 7
Jordy Clasie 8
Victor Wanyama 7
Steven Davis 7
Dusan Tadic 9
Sadio Mane 9
Shane Long 8
Jay Rodriguez (for Tadic, 86)
Oriol Romeu (for Clasie, 58) 6
James Ward-Prowse (for Davis, 72) 6
Joe Hart 5
Pablo Zabaleta 4
Nicolas Otamendi 4
Eliaquim Mangala 4
Aleksandar Kolarov 4
Samir Nasri 5
Fabian Delph 3
Raheem Sterling 4
Kelechi Iheanacho 8
Wilfried Bony 4
Jesus Navas (for Delph, 63) 6
Martin Demichelis (for Fernandinho, 58) 5
Mane: The hat-trick meant he edged out team-mate Tadic, who set up three of Southampton's four goals.
MOMENT OF THE MATCH:
Mane's first goal thanks to Tadic's expert slide-rule pass. Saints' second goal in three minutes effectively sealed their victory midway through the first half.
VIEW FROM THE BENCH:
City paid the price for their eight changes from Tuesday's goalless draw with Real Madrid in the Champions League semi-final first leg in Manchester. The visitors' second string was simply not up to task, nor fully invested in the challenge, to produce the victory boss Manuel Pellegrini was after. Ronald Koeman, on the other hand, will be delighted to have taken advantage.
MOAN OF THE MATCH:
Manchester City spent ?8million on Delph, and the former Aston Villa man was almost comically out of his depth at St Mary's. City have shelled out far bigger money on several stars yet to justify their presence or price tag, but signing Delph appears little more than a box-ticking exercise to boost the quotas of English stars on the Etihad outfit's books.
WHO'S UP NEXT?
Tottenham v Southampton (Premier League, Sunday May 8)
Real Madrid v Manchester City (Champions League, Wednesday May 4)