The Southampton manager and the Manchester United boss have a peculiar relationship.
Southampton and Manchester United face off at St Marys in the Premier League this week, in a shoot-off for the fourth spot in the Premier League.
While the two teams are not renowned for a special rivalry, although Southampton used to enjoy the odd victory in the days of Matt Le Tissier, the two managers are certainly not best of friends.
The pair first linked up back in 1998 - when Louis van Gaal appointed him as the assistant coach at Spanish giants Barcelona.
That ended in the year 2000. Koeman jumped ship to takeover at Eredivisie side Vitesse Arnhem, with Van Gaal's blessing, while LvG resigned at the end of the season.
Koeman went on to become a great success at Vitesse, earning himself the opportunity to work at Ajax as head coach and it was here the two met again.
Van Gaal was coming off the back of two disappointing management spells. He failed to take Holland to the World Cup in 2002 and was sacked by Barcelona in 2003, with the Catalan giants hovering above La Liga's relegation zone. But he was appointed technical director at Ajax in 2004, which is where he and Koeman finally came to blows.
Apparently he watched Koeman's training sessions at a camp in Portugal like a headmaster assessing a pupil - which did not go down well with Koeman, now a successful manager in his own right.
Van Gaal criticised Koeman in his autobiography saying he didn't have the players in hand and that he had to separate Zlatan Ibrahimovic and Rafael van der Vaart from a fight because Koeman was 'too weak'.
He sold Zlatan, who had become a problem, but Koeman complained in the press and shortly after Van Gaal stepped down as technical director citing 'internal conflict'.
But the rivalry appears to go back even further then that.
This picture is from 1982. It shows Ronald Koeman, then a young player making his name at FC Groningen sprawled on the floor and Louis van Gaal, an established individual with Sparta Rotterdam at the time, telling him in no uncertain times to get up from the ground.
A scary moment we imagine for sure.