Charlie Austin scored a fairy tale winning goal just eight minutes after coming on for his Saints debut to hand his new team a dramatic 1-0 win against Manchester United.
The £4m man, who only joined from QPR a week previously, stepped off the bench on 79 minutes at Old Trafford with the game deadlocked, but soon put away a clinical header with his first chance in a Saints shirt to seal a memorable win.
Many had predicted a game short on chances and the opening stages certainly backed up that theory.
Both sides had numbers behind the ball as they sparred for superiority.
Koeman tried to ensure Saints would have the chance to be a threat by slightly altering the formation from the last couple of games with two players rather than one, in this case Sadio Mane and Dusan Tadic, playing behind Shane Long with the wing backs and defensive midfielders helping out the three centre halves.
Anthony Martial was the first player to get a shot off, capitalising on the Saints defence backing off, but from 20 yards out he dragged it well wide.
Daley Blind tried his luck in similar circumstances but from even further out. His shot was well struck but straight at Fraser Forster who made a comfortable catch.
Saints were complaining to Mike Jones on 16 minutes when Victor Wanyama went down clutching his face after being caught by a trailing elbow from Marouane Fellaini as he jumped for a header but the referee saw nothing wrong.
Saints were again guilty of giving a player too much space, Wayne Rooney on this occasion, but once more a shot from outside the box failed to hit the target.
Cedric Soares had a first shot of the match for Saints on 24 minutes but it was sliced well over the bar.
Rooney had another shot from distance, this one going wide of the far post.
Although they hadn’t created much, Saints were looking so assured and unruffled.
Their defence was rock solid and they were pressing Manchester United, with Jordy Clasie in particular tireless.
Clasie produced a first half chance for Saints as he whipped in a left wing free kick which found Wanyama at the far post but he headed over under pressure from Rooney.
Saints came so close to crafting a great opening on 36 minutes as Mane played a one-two with Tadic. The Serbian’s lofted pass picked out Mane’s run perfectly but as he tried to bring the ball down to shoot in the area it bobbled off his knee and David De Gea was able to smother it.
The Old Trafford crowd stuck behind their side for most of the first half but were clearly starting to lose their patience in the final stages.
Every time Saints pressed in the United half they were winning the ball with the Red Devils conceding possession with ease.
Jesse Lingard had one final effort for United in the first half but it deflected wide to leave Saints going in at the break by far the happier side with Koeman’s tactics working perfectly.
United made a change before the restart with Juan Mata replacing Fellaini.
It did invigorate them as they were keeping the ball much better and starting to ask a few questions of the Saints defence as they kept the pressure on.
Saints knew they had to weather the initial storm and if they could avoid conceding they might turn the crowd and get on top again.
United were forced into a second substitution on the hour mark as Matteo Darmian hobbled off to be replaced by Paddy McNair after an unfortunate clash with Long. Darmian made an excellent clearing header as Mane broke quickly down the right and looped a far post cross in but collided with Long after landing.
Wanyama had a great chance for Saints two minutes later as Ryan Bertrand’s deep corner found him peeling off at the far post but from just outside the six yard box he headed wide.
Saints made a double change on 67 minutes with Oriol Romeu and James Ward-Prowse replacing Clasie and Tadic.
Romeu tried to have an instant impact with a control on the chest and volley from outside the area but it went well over.
Charlie Austin was handed a Saints debut as he came on for Mane on 79 minutes as Koeman put two up front.
Ward-Prowse whipped in a free kick moments later that picked out the head of Long but from eight yards out he could only guide it over the bar.
United made a final change with four minutes remaining as Adnan Januzaj came on for Cameron Borthwick-Jackson.
The fairy tale came true on 87 minutes as Austin scored a dramatic goal.
He won a free kick in the right corner after strong hold up play.
Ward-Prowse whipped in a right footed outswinging cross.
Austin stood pretty much still in the area as United bodies charged ahead, leaving him totally unmarked eight yards out in the centre of goal.
He showed exactly why Saints signed him as he nailed a clinical header low past De Gea for 1-0.
Saints almost sealed the win in stoppage time as Romeu shot from a tight angle but De Gea got down well to save.
Januzaj threatened very late on but fired a half volley wide of the target as Saints grabbed a brilliant win with Austin a hero already.