It was during one of their last meetings in England, rather fittingly, that Louis van Gaal and Ronald Koeman sparked the showdown that had the most lasting effect on their relationship. It certainly fostered the frostiness that has been so discussed before Manchester United’s trip to Southampton on Monday night.

Back in the summer of 2004, Ajax were on a pre-season trip to England, with Koeman as head coach and Van Gaal as technical director. At one point, the latter took Koeman aside to discuss the coach’s performance. This was despite the fact Koeman had just won his second Dutch title, and that such a discussion was not part of Van Gaal’s job description.

Koeman abrasively pointed this out: “I don’t know if you’ve read my contract, but the only person I have to justify myself to is the general manager.”

“From that moment on,” Koeman (left) wrote in his autobiography, “it was a real battle between us.”

On Monday, almost a decade after that dispute forced Van Gaal (right) out of Ajax, we will see a real showdown between them. From a situation where one wanted to tell the other how to do his job, all the talking will now be on the pitch.

That’s just as well, because neither manager was willing to discuss their relationship in the past week. “It’s private”, was the message.

Their feud has always been more complex than mere rancour, and their careers have crossed paths at several points. Van Gaal gave Koeman his first coaching role, bringing him to Barcelona in 1998. One of the Camp Nou hierarchy had actually warned Van Gaal not to bring in the former centre-half, saying many had found Koeman “egotistical” during his previous time at the club. Van Gaal waved that aside but, according to some in Spain, gradually realised they were right.

When Van Gaal was appointed Ajax technical director in 2004, Koeman – who had by then been head coach for three years – received a text laced with sarcasm from his brother Erwin.

“Good luck with your new TD,” it read. The problem was that Van Gaal wanted to be so much more than a “TD”, despite his public protestations that he did not wish to coach beyond the age of 55.

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It all became so obvious after one particular clash of egos. Koeman had been assured the technical director would not speak to the team but, perched conspicuously on a white seat at the edge of the pitch during a training game, Van Gaal couldn’t help himself.

He called over a young Zlatan Ibrahimovic and told him he wasn’t attacking the near post enough for a centre-forward. The Swede was furious, but Van Gaal was proved right when Zlatan scored by precisely following his instructions.

As the ball hit the net, Van Gaal leapt up and celebrated the goal ostentatiously, perhaps to prove that his coaching had been correct. Koeman was “livid” and described it as “very embarrassing”.

Van Gaal ended up stating publicly that the team were not keeping to the Ajax philosophy. This, along with the sale of Ibrahimovic to Juventus against Koeman’s wishes, was “a final straw”. The club sided with Koeman and he and Van Gaal stopped talking.

The difference of opinion over Ibrahimovic reflected the differences between the managers, and perhaps why one has been more successful than the other. Van Gaal let the Swede go because he would not accept total control, whereas Koeman was more laissez-faire.

That was further emphasised when their career paths crossed again. Koeman directly succeeded Van Gaal as AZ Alkmaar manager in 2009, but couldn’t come close to matching the latter’s feat of winning the league. The players found Koeman’s coaching too loose and he was sacked within six months.

It wasn’t long, however, before some former Dutch players were expressing surprise about Van Gaal’s career. Marco Van Basten was one of many who were shocked by Van Gaal’s return coach of the Dutch side in 2012, especially as a then resurgent Koeman was one of many supposed to be above him on the shortlist.

Manchester United manager Louis van Gaal (GETTY IMAGES)

Van Gaal proved the doubters wrong by reaching the World Cup semi-finals – but not without Koeman’s help. As Dutch manager, Van Gaal had wanted to call up Feyenoord’s Bruno Martins Indi and Stefan de Vrij. Dutch protocol meant he had to discuss it with the club coach – Koeman.

The Dutch Under-21 coach Cor Pot brokered a meeting and, from that point on, their relationship has improved.

Van Gaal also took much more than Koeman’s players to the World Cup – he took his system.With seven games left last season and Feyenoord slipping down the league, Koeman changed to a formation virtually unused in the country: five at the back. The switch had an instant effect, and Van Gaal was there with Robin van Persie to watch one of Feyenoord’s best games, a 2-0 win over PSV Eindhoven.