Charlie Austin came on to score the winner against Manchester United - a dream for any manager who has signed a new striker and made the call to bring him on to change a game.
And this wasn't the first time Koeman has beaten Louis van Gaal with a visiting Southampton side, of course.
Reacting on Twitter after the win, Koeman was emphatic:
Responding to the Dutchman, meanwhile, Southampton fans were just as emphatic, praising the 52-year-old and showing their support for a Saints side that have bounced back superbly from a poor run of form.
But there was also a response from a 'jealous' Manchester United supporter - who also lauded Koeman for his work.
The Premier League table now looks a totally different prospect for Southampton. They are up to eighth and are just four points off United - who are fifth...
@RonaldKoeman What a match! Stifled United the whole game and never game them a chance. Couldn't have asked for a better debut for Austin.— BenG (@highoncloud09) January 23, 2016