Charlie Austin made a dream Southampton debut by heading an 87th-minute winner to sink a dire Manchester United 1-0 at Old Trafford.
Austin returned to the Premier League this week as Saints pounced for the former QPR man, and he stepped off the bench late in the day to illuminate an otherwise dull encounter that did nothing to enhance Louis van Gaal's standing with United supporters.
The home faithful did not hold back displeasure as their side fail to score before half-time for an 11th consecutive home game and Juan Mata's half-time introduction brought no more than sporadic flickers of life in attack.
Southampton carried a set-piece threat before Austin entered the fray in place of Sadio Mane in the 79th minute and his decisive goal owed something to an over-eager challenge from returning United substitute Adnan Januzaj.
Making his comeback after a fruitless loan stint at Borussia Dortmund, Januzaj brought down Austin and the former Burnley striker planted a fine header beyond David de Gea from James Ward-Prowse's unerring right-wing delivery.
United slip five points behind fourth-placed Tottenham, while Southampton move up to eighth, having claimed three consecutive Premier League wins for the first time this season.
Cameron Borthwick-Jackson made his first Premier League start at left wing-back after Ashley Young sustained a groin injury, with Van Gaal switching to 3-5-2 to match his opponents' shape.
Two goals in the 3-0 win over West Brom were not enough to keep Ward-Prowse in the Southampton XI, the midfielder joining Austin on the bench as Jordy Clasie and Dusan Tadic earned starts.
Mane had a desperate first half's best sight of goal in the 37th minute, but his touch and footing deserted him in the United area after playing a neat one-two with Tadic.
That let-off did not quell discontent in the stands - the now familiar lament of "attack, attack, attack" ringing out - and Jesse Lingard's deflected stoppage-time effort spinning wide could not prevent half-time jeers.
Van Gaal responded by sending on Mata in place of the ineffective Marouane Fellaini in an attempt to spark some creativity.
Mata and Wayne Rooney combined in the desired fashion after 54 minutes, although Anthony Martial could not get a shot off and Southampton goalkeeper Fraser Forster gratefully grasped the ball.
Van Gaal defensive injury woes worsened when Matteo Darmian came off worse in a 58th-minute collision with Shane Long - the Republic of Ireland striker heading a subsequent corner wide under pressure from the Italian's replacement, Paddy McNair.
A goalless draw seemed inevitable before Austin's accurate finish thrillingly grabbed the bragging rights for Koeman over his old rival Van Gaal after just eight minutes on the field.
Another Southampton sub, Oriol Romeu drew a sharp save from De Gea as the visitors threatened further misery before Januzaj failed to atone for his error by fizzing an injury-time shot wide
The final whistle brought further ire for Van Gaal, as a second-successive 1-0 home defeat to old foe Koeman concluded the latest chapter of an increasingly unpopular tenure.