Manchester United have been left stunned at the Stadium of Light as Sunderland deservedly clinched three points in front of a delighted home crowd.
January signing Wahbi Khazri opened the scoring for the hosts, before Anthony Martial netted the equaliser to make it all square at the break.
A David De Gea own goal, however, sealed victory for the Black Cats, boosting their fight to avoid relegation and leaving the Red Devils six points adrift of the top four.
With fourth-placed Manchester City six points ahead, United headed into the game knowing that no less than victory would be required if they were to sustain a challenge for a Champions League spot.
Despite this, they got off to a terrible start.
With less than three minutes on the clock, winter window arrival Wahbi Khazri, much to the shock of even the home fans, whipped the ball into the box from the left flank and watched as his curling delivery passed everyone by, including David De Gea to open the scoring.
Van Gaal’s side almost hit back immediately, with Juan Mata playing Wayne Rooney in, but the striker’s effort was blocked by Patrick van Aanholt at close-range.
The Black Cats responded to their one-goal lead as any relegation-threatened predictably would; dropping deep, almost encouraging their opponents to press forward.
It looked to be more of the same for United, with much possession and little penetration; half an hour in, however, and the pressure was ramped up.
Juan Mata came close on the edge of the box, exquisitely meeting the ball on his majestic left foot, but his effort fell comfortably into the midriff of Vito Mannone.
It was a frustrating first half for the visitors, but shortly before the break the pressure finally paid off, with Anthony Martial, again playing out of position on the wing, grabbing the equaliser.
Mannone initially denied a powerful strike from Mata, only to see his save fall into the path of Martial and the Frenchman made no mistake, beautifully dinking the ball over the Italian goalkeeper from an acute angle.
Donald Love later made his debut, replacing Matteo Darmian, who left the field clutching his shoulder, adding to a lengthy injury list.
The Red Devils were desperate to grab a second before the referee blew his whistle, with Mata, Rooney and Martial causing plenty of problems for Allardyce’s men.
The hosts, though, held on, leaving all to be fought for in the second half.
It was more of the same as the final 45 minutes kicked off, with United continuing to dominate play.
Sunderland, though, looked dangerous on the counter-attack, and De Gea’s goal was threatened more than he would have thought acceptable.
Memphis Depay, who seems to have lost the trust of his manager over the past few months, replaced the ineffective Jesse Lingard on the hour mark, and soon after his arrival the Netherlands international almost set up his side’s second.
Having surged into the box, from the left wing, he squared the ball to an unmarked Wayne Rooney only for a superb block by Jack Rodwell denying the England captain what looked to be a certain goal.
With time running out, United seemed to be lacking in ideas.
And true to the most unpredictable of seasons in Premier League history, Sunderland took the lead with late on, with another of the hosts’ new signings proving crucial.
Lamine Koné rose highest from the corner, and his header crept under the legs of De Gea which, sadly for the Spaniard, goes down as an own goal.
Van Gaal turned to Under-21 star Will Keane, bringing off Morgan Schneiderlin.
Depay cried for a penalty in the final minutes, believing his effort on the half-volley had illegally been blocked by the arm of John O’Shea.
His pleas were swiftly ignored by the officials, however, and the resulting corner was dealt with by Sunderland.
Again, it’s back to the drawing board for the under pressure Van Gaal.