Manchester United began their season with a whimper as Gylfi Sigurdsson gatecrashed Louis van Gaal's Barclays Premier League bow at Old Trafford.
New United captain Wayne Rooney thought he had grabbed a point for the hosts with an overhead kick following Ki Sung-yueng's opener.
But Sigurdsson, making his second debut for the Swans, beat David De Gea from 15 yards to give Garry Monk's team a shock 2-1 win.
Van Gaal said going into the match he was not nervous about the task of bringing the glory days back to Old Trafford following last year's dismal showing under David Moyes.
But the Dutchman will surely have reservations about his squad's ability having seen what transpired during his first taste of Premier League action.
Moyes may have gone, but the same problems remain for United.
They lacked a cutting edge up front and their passing was sloppy at times.
But the main concern for Van Gaal will be in defence. Both Sigurdsson and Ki were unmarked when they beat De Gea.
There was a clear lack of organisation at times in defence, with Phil Jones and Chris Smalling both looking out of their comfort zone on several occasions thanks to Swansea's enterprising play and neat passing.
United were crying out for the experienced heads they lost this summer.
The defeat to Swansea, who are among the favourites to go down, suggests Van Gaal desperately needs defensive reinforcements, and the quicker he moves in the transfer market, the better.
A wave of noise greeted Van Gaal and his players as they emerged from the tunnel to signal the start of a new chapter in United's rich history.
Any of the 75,339 fans inside Old Trafford expecting a lightning start to this Premier League season were left disappointed as a dull opening 15 minutes ensued.
Swansea did manage to test United's new-look three-man defence on a couple of occasions.
Wayne Routledge raced on to Jonjo Shelvey's through-ball, but David De Gea came to the rescue.
Routledge then gave Jones the slip down United's right, but his cross was cut out just before Wilfried Bony could convert.
Rooney led by example as he tried to drag United into the game. The new United captain tore around the pitch, but his two first-half attempts on goal were weak.
Jesse Lingard, making his full United debut, was replaced by Adnan Januzaj after injuring his right foot in a challenge with Ashley Williams.
Finally, with 24 minutes on the clock, Swansea goalkeeper Lukasz Fabianski was tested for the first time when he saved Jones' blistering shot after a clever one-two with Juan Mata.
Then, four minutes later, Old Trafford fell silent as Swansea took the lead.
Blackett was drawn out of position by Nathan Dyer, who laid the ball off to Sigurdsson. The Iceland international squared to Ki, who fired the ball past a diving De Gea.
Bony played an important part in the goal, blocking Jones as he rushed out to challenge the South Korean goalscorer.
Van Gaal's men rallied briefly, but in truth United were hanging on towards the end of the first half.
Van Gaal sought to inject life into his team by bringing Nani on for the ineffective Javier Hernandez, who started up front ahead of the unfit Robin van Persie.
Within eight minutes of the restart the hosts were level. Jones climbed well to flick on Mata's corner and Rooney acrobatically swept the ball past Fabianski.
Raucous celebrations ensued on the United bench, but Van Gaal simply recorded the goal on his notepad and looked up with a blank expression.
He wanted a second, but Swansea would not relent.
Sometimes the Welsh side went too far in their attempts to stop the hosts, who were now in full flow.
Neil Taylor went in to the book for hacking down Januzaj and Mike Dean also issued a caution to Shelvey for kicking the ball away following a late tackle on Mata.
Rooney turned Williams with an exquisite bit of skill, but the Swansea captain recovered to deny the England striker.
Rooney then came agonisingly close to putting United ahead with a brilliant free-kick.
After being hacked down by Jordi Almat on the edge of the area, the United captain curled a looping shot up and over the wall, but unfortunately for Van Gaal, the ball struck the frame and bounced wide.
Rooney cursed his luck, but worse news was to follow for the United captain.
In the 72nd minute, Routledge mis-hit Jefferson Montero's cross. Fortunately for Swansea, the ball fell at the feet of Sigurdsson, who bundled the ball past De Gea.
United were stretched at the back as they looked for a second equaliser. Bony slipped in behind, but Jones got back just in time.
With five minutes left, the fans behind the United dugout started filing out. They had seen enough.
United had a late penalty appeal turned down, but Dean waved play on and Swansea held on for their first league win at Old Trafford.