Gylfi Sigurdsson's smart finish saw Swansea City claim a surprise 2-1 win at Manchester United as the Louis van Gaal era started in defeat.
Sigurdsson bagged the winner 18 minutes from time on Saturday as Van Gaal – tasked with improving the fortunes at Old Trafford after David Moyes' disastrous spell in charge last term – endured a day he will want to forget in his opening Premier League fixture.
The Dutchman's 3-4-1-2 system did not appear to suit United despite being used in pre-season and they found themselves behind when Ki Sung-yueng scored from 20 yards just before the half-hour mark.
Van Gaal, known for his tinkering, swapped to 4-4-1-1 in the second half and it appeared to be the tonic for United's response, led by new captain Wayne Rooney, who levelled with a clever overhead kick eight minutes after the break.
Rooney saw an outstanding free-kick crash against the post with 24 minutes left and Old Trafford was then stunned when Sigurdsson – Swansea's main signing of the close-season – finished well after Wayne Routledge's cut-back.
The result is sure to put early pressure on Van Gaal and is likely to see further spending from United in the transfer window, as they look to recover from last term's seventh-place finish.
The victory was Swansea's second at Old Trafford in 2014 – and second ever - as the Welsh club also won a third-round FA Cup tie by the same scoreline in January.
Van Gaal gave 20-year-old defender Tyler Blackett a Premier League debut, while Spanish midfielder Ander Herrera and Jesse Lingard also made their top-flight bows.
Lingard started in the unfamiliar position of right wing-back and was nearly caught out just six minutes in, when Routledge drifted across him untracked, but the Swansea man's first touch meant that Jonjo Shelvey's clever ball in behind was squandered.
Sigurdsson – back at Swansea for a second spell, this time permanently, after signing from Tottenham – saw his 25-yard shot in the 12th minute fall awkwardly for United goalkeeper David de Gea, but the Spaniard did well to smother it.
Lingard was then injured in a challenge with Ashley Williams, and replaced by Adnan Januzaj, before the hosts eventually got going, with Lukasz Fabianski – the only debutant for the visitors from the start – needing to be alert to keep out Phil Jones' deflected cross and Juan Mata's inswinging effort.
Swansea were able to convert one of their chances, though, as Ki delighted the travelling fans in the 28th minute.
The South Korea international was played in by Sigurdsson, who initially linked with Nathan Dyer, and he hit a sweet left-footed effort that curled into the right corner from 20 yards.
Van Gaal's men failed to mount a significant response before the break and as a result, he replaced the ineffective Javier Hernandez with Nani and changed systems.
Rooney then produced the equaliser eight minutes into the second half, showing great anticipation to thrash Jones' near-post flick-on home from four yards in sensational style.
The England star continued to threaten, with Williams required to deny him with a last-gasp tackle, before he was then thwarted by the woodwork as his wicked 25-yard set-piece left Fabianski rooted to the spot, but crashed back off the post.
And Swansea made the hosts pay for their profligacy as a quick free-kick, taken by Wilfried Bony, set them free.
A cross from substitute Jefferson Montero made its way through to Routledge at the far post, and although he slipped, his ball fortunately fell for Sigurdsson, who hammered an effort through a sea of bodies that was too hot to handle for De Gea.
United, so often saved by late goals, rarely looked like levelling, but were denied a penalty despite Januzaj's cross striking the arm of Angel Rangel in injury time, as Swansea claimed an excellent win against the odds.
|Coach: L. van Gaal|
for J. Lingard 24'
for J. Hernández 46'
for Ander Herrera 67'
|Ventura: Buffon welcome in Italy coaching set-up|
|Marseille stump up for 160 fans´ Dijon stays|
|Gabriel will only go on loan to a club on Inter´s level, says agent|
|Bayern do not bow to economic force - Hoeness|
|All clubs would want Sturridge, says Bilic|
|Seaman urges Arsenal to spend big on keeping Sanchez and Ozil|
|Bravo happy to accept criticism for City showings|
|Origi reveals United, Bayern snubs|
|Cavani: I´d like to play with Griezmann one day|
|Aguero is not aggressive, claims Guardiola|
|Ibrahimovic wants United to be ruthless|
|Sanchez can be Arsenal´s new Henry - Wenger|
|Andre Gomes can play in Busquets role, insists Luis Enrique|
|Aguero´s dangerous tackle scared me, says Conte|
|Guardiola: We are killing the players|
|Bournemouth announce plans for new stadium|
|Former Bayern Munich and Real Madrid star Ze Roberto to play beyond 43rd birthday|
|Barcelona friendly awaits new Chapecoense coach Mancini|
|Arsenal are privileged to have Sanchez and Ozil - Monreal|
|Watzke denies agreement to discuss Aubameyang transfer with Madrid|
|Paris Saint-Germain v Nice: Champions hunting down shock leaders|
|Giggs tells Rashford how to end his goal drought|
|Manchester United without Shaw, Smalling for Tottenham clash|
|Willems inks PSV renewal|
|Costa and Conte scoop Premier League awards|
|Conte detained on suspicion of assault|
|Strootman ban for derby ´simulation´ overturned|
|Mourinho demands more goals from Rooney|
|Bobo preserves Sydney´s unbeaten start in Cup final rematch|
|Luis Enrique: I don´t dare predict if Messi will renew|
|Mourinho admits changing style for United|
|Sanchez talk is disrespectful, says Conte|
|Sassuolo 0 Genk 2: Visitors seal top spot|
|Sterling back for City as Guardiola predicts tough Leicester test|
|Mourinho still the ´Special One´ for Pochettino|
|Matic return leaves Conte with Fabregas dilemma|
|Guardiola expects greater impact from City youngsters next season|
|Klopp defends Karius after Bournemouth blunder|
|Pirlo visits Chelsea - but Conte is not signing him|
|Ronaldo, Modric rested for Deportivo clash|
|Tuchel hails returning Reus|
|Klopp has ´no idea´ on Sturridge return|
|´My players are intelligent and good people´ - Zidane on Ronaldo´s taxes|
|Atletico to play at ´Wanda Metropolitano´|
|Real Madrid to rest Benzema and start Morata against Deportivo|
|Ancelotti rules out Draxler bid as Bayern prepare for barren transfer window|
|Ighalo calls on Watford to sign Mikel|
|Pardew wants defender and striker to bolster Palace survival push|
|Moyes: M´Vila doesn´t have the fight for Sunderland|
|Leipzig only built to earn Red Bull money - Watzke|
|Shenhua director blasts CSL imports... but desperate for Tevez deal|
|Ribery: Bayern my last club in Europe|
|Hughes targets Arsenal fans to help Stoke end dismal away run|
|83 suspects, 98 clubs ´impacted´ in football abuse probe|
|Fowler pleads with Liverpool to keep ´top quality´ Sturridge|
|Reus ´100 per cent different´ on comeback|
|Ibrahimovic ´super happy´ for rejuvenated Mkhitaryan|
|Vertonghen: Football needs Mourinho|
|Title triumph went to Leicester´s heads, admits Ranieri|
|Conceicao named new Nantes coach|
|LaLiga lead means nothing - Zidane|
|Chapecoense add two stars to badge in wake of air disaster|
|Blatter accuses Infantino of lack of respect|
|Austin injury compounds Puel´s misery|
|Gestifute publishes further evidence of ´Ronaldo tax compliance´|
|Pioli eyes further Inter improvement|
|´I feel s***´ - Van Dijk blasts Southampton after Europa League collapse|
|Mourinho marvels at Mkhitaryan mentality after Zorya stunner|
|Southampton 1 Hapoel Be´er Sheva 1: Late Van Dijk leveller not enough as visitors progress|
|Inter 2 Sparta Prague 1: Eder double sends hosts out on a high|
|Vieira wanted Arsenal coaching opportunity|
|Vieira hopes MLS experience puts him in frame to replace Guardiola|
|Mkhitaryan: I waited a long time for this|
|Zorya 0 Manchester United 2: Mkhitaryan magic sends Mourinho´s men through|
|Ranieri urges Leicester players to fight for their futures|
|Cellino banned for 18 months over McCormack transfer|
|Koeman demands reaction from dropped Barkley|
|Tevez unsure on Boca future amid China speculation|
|Bradley vows to fight for his job at Swansea|
|Koeman speaks out on Depay, Schneiderlin links|
|Wenger unconvinced by World Cup expansion plans|
|Kovac brothers sign new Frankfurt deals|
|Selling Sturridge would be a mistake, says Rush|
|UEFA opens disciplinary proceedings against Besiktas, Dynamo Kiev|
|Filipe Luis suffers thigh tear|
|Kashima Antlers 2 Auckland City 1: Late Kanazaki header the difference|
|Suarez´s Barca renewal already done, says Bartomeu|
|Barcelona invite Chapecoense for Joan Gamper Trophy|
|Arsenal would have a chance against Real Madrid, insists Wenger|
|FIFA chief Infantino calls for zero tolerance on child abuse|
|Sanchez will not leave Arsenal for China - Wenger|
|Dybala delighted with Juventus return|
|Rashford desperate to learn from Ibrahimovic|
|Weigl not ready for Real Madrid - Tuchel|
|Why Zidane must drop Ronaldo at the Club World Cup|
|Barton: They built me up into a Neymar-Messi type|
|Shakhtar aware of Barca interest in Srna - Fonseca|
|Mets hitting coach believes Tebow can make it to MLB|
|Aubameyang reiterates Madrid dream after Bernabeu show|
|Zidane praises Benzema after 50th Champions League goal|
|Aubameyang deserves 99 per cent of credit - Reus|
|Neymar: There´s only one Ballon d´Or winner and it´s Messi, not Ronaldo|
|Magiera hails Legia victory as for ´all of Poland´|
|Tuchel thought Aubameyang had blown chance to equalise|
|Pochettino confident Spurs are back on track|
|Santo hails Porto´s mature performance|
|Lyon proved we can compete with Europe´s best - Genesio|
|Slutsky unsurprised by apparent Premier League interest in Dzagoev|
|Jardim frustrated as Monaco finally lose|