United were hoping to turn the page on their mid-week Champions League exit, but Junior Stanislas put Bournemouth in front straight from a corner after 100 seconds and although Marouane Fellaini equalised, former United trainee King punished slack defending at a corner to claim a 54th-minute winner.
United could have gone level with new Premier League leaders Manchester City, but their third defeat of the campaign left them three points off the pace in fourth and at risk of falling six points behind Leicester City, who host Chelsea on Monday, generating yet more bad press for the floundering Van Gaal.
It was a second stunning win in eight days for Eddie Howe's promoted Bournemouth following their 1-0 triumph at champions Chelsea last weekend and took them three points clear of the relegation zone.
With injuries forcing Van Gaal to field a defence featuring Cameron Borthwick-Jackson, 18, and 20-year-old Paddy McNair, United fell behind inside two minutes when Stanislas's wind-assisted corner curled straight in.
Fellaini equalised in the 24th minute, bundling the ball home at the second attempt after Memphis Depay had been thwarted by Bournemouth goalkeeper Artur Boruc.
But after Jesse Lingard had gone off injured, King restored Bournemouth's lead early in the second half when he gave Daley Blind the slip and applied an adroit finish to Matt Ritchie's right-wing corner.
Substitute Glenn Murray, who scored a late winner at Chelsea, twice squandered chances to give Bournemouth breathing space, blazing over from close range on each occasion, but in the end it mattered not.
Kelechi Iheanacho's fortuitous stoppage-time goal had earlier given Manchester City a 2-1 win over managerless Swansea City that sent Manuel Pellegrini's side above former leaders Leicester on goal difference.
Swansea, who sacked Garry Monk on Wednesday, looked to have earned a point when Bafetimbi Gomis scored a 90th-minute equaliser, only for Yaya Toure's shot to hit team-mate Iheanacho and loop in two minutes later.
“I think the three points were very important, more than the way we played,” said Pellegrini.
“Swansea played very well and it's not the way we want to win. Now we have a week to recover -- we have played too many games with 13 or 14 players.”
Former Swansea striker Wilfried Bony put City in front in the 26th minute at a rain-lashed Etihad Stadium by meeting Jesus Navas's left-wing corner with a near-post header.
Gylfi Sigurdsson thought he had equalised in the 84th minute, but his driven effort was contentiously ruled out as Gomis, on as a substitute, had collided with City goalkeeper Joe Hart a split-second earlier.
Swansea did level minutes later, Gomis running onto Federico Fernandez's slide-rule pass and slamming a shot past Hart at his near post, only for City to wrest back the advantage in the cruellest fashion.
Swansea, who have won only once in 12 league games, remain a point above the relegation places.
“We are bitterly disappointed to come away empty-handed, but that performance is the standard,” said Swansea's caretaker manager Alan Curtis.
“We have to maintain that and if we do, I am 100 percent confident we will get out of trouble.”
Elsewhere, Crystal Palace drew level on points with fifth-place Tottenham Hotspur, who host Newcastle United on Sunday, after Yohan Cabaye's 38th-minute tap-in earned them a 1-0 home win over Southampton.
Everton remain in ninth place after squandering a chance to close on the leading sides in a 1-1 draw at Norwich City.
Romelu Lukaku headed in a cross from Gerard Deulofeu in the 15th minute to put Everton ahead, but the visitors spurned a string of chances and Wes Hoolahan scrambled home a Norwich equaliser early in the second period.
Watford moved up to seventh place, four points below the Champions League spots, after Odion Ighalo's 10th goal of the campaign secured a 1-0 win at Sunderland, while West Ham United drew 0-0 at home to Stoke City. – AFP
Original source: Bournemouth sink United