Louis van Gaal’s time at Manchester United has so far been a rollercoaster of emotions as the club struggles to assert the dominance they once held upon the English Premier League under Sir Alex Ferguson.

However, few can argue with the pedigree of the 64-year-old former P.E. teacher and his managerial campaigns in Holland, Spain and Germany have all proved bountiful.

The coaching style of the Amsterdam-born van Gaal is perhaps the most infamous aspect of his career in management and his philosophy and playing style echo around European football whilst his legacy will long continue to flourish through the likes of former apprentices Pep Guardiola, Jose Mourinho and Luis Enrique.

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Yet it is the relationship with former understudy and current Southampton manager Ronald Koeman that currently remains at the forefront of his current legacy and could yet prove to be one of the most controversial relationships between two individuals in football management.

Following a prestigious playing career that saw Koeman win almost every trophy and collect a wealth of personal accolades, the Zaandam-born former midfielder made a quick transition from his playing career into management after accepting a role on the Netherlands coaching staff under Guus Hiddink alongside Johan Neeskens and Frank Rijkaard ahead of the 1998 World Cup shortly after retiring with Feyenoord.

And after helping the Netherlands to a fourth-place finish at the World Cup, Koeman was approached to join the backroom staff of van Gaal’s La Liga winning Barcelona as the former Ajax manager looked to continue his successes at his new club in both the domestic league and in Europe. Koeman joined not only van Gaal on the bench but also promising Portuguese coach Jose Mourinho who had previously helped Sir Bobby Robson before his departure from Catalonia.

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However, despite all looking well on the surface of things, van Gaal struggled to assert his philosophy at the club and his infamous fallout with the club’s star player Rivaldo as well as his constant clashes with the Spanish media resulted in the Dutchman eventually departing back for his home nation.

And after his bitter departure from the Catalan club, it would be a number of years before the former master and apprentice would meet again as Koeman made his bow managing domestically in the Netherlands whilst van Gaal took over the Dutch national team before once again failing to gather any sort of success with Barcelona.

Eventually, the pair joined forces once again at Ajax as Koeman, who had impressed during his brief spell at Vitesse where he managed to guide them to a UEFA Cup spot with very limited resources was identified as the man to help turn around a struggling Ajax side – and he managed just that, guiding them to a domestic double in his first season whilst winning the league again in 2003/04.

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During this time, van Gaal had once again departed Barcelona on bad terms and ended up taking some time out of management before being approached in 2004 by the Ajax hierarchy to take the role of director of football. In the eyes of the board at the club, they believed that the pair who had previously worked together in Spain, coupled with the duo’s outstanding knowledge of Ajax could help propel the side back to the top of Dutch football.

And it was the return of the Dutch general that really saw the relationship between the pair disintegrate entirely.

As described by Elko Born for FourFourTwo, tensions began to boil when van Gaal began to stick his nose in the business of the first-team coach’s business and although his duties were to deal with scouts and identify transfer targets, he would instead often look to intervene on the training ground and in turn, Koeman began to believe that his fellow colleague was looking to undermine him – rather than assist in successfully running the club.

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His insistence on arranging meetings regarding tactical issues would only help to further annoy Koeman and eventually the final straw was drawn when van Gaal sanctioned the sale of star striker Zlatan Ibrahimovic to Juventus on transfer deadline day without lining up a replacement.

The pair’s relationship was never recovered and as a result, the side suffered both in the league and in Europe as they fell behind bitter rivals PSV in the league whilst also being eliminated from the UEFA Cup by Auxerre.

Eventually, the board decided that they could not keep on the pair for their fiery altercations and Koeman shortly departed the club after van Gaal was removed.

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Nowadays, the duo find themselves facing off once again after managing to stay out of each other’s way for almost ten years. However, Koeman knew that once he had taken the job at Saint Mary’s that the questions would soon follow. Speaking to The Independent shortly after his appointment, Koeman said:

We had a problem because I was the coach of Ajax and he [van Gaal] came in as technical director. We had some problems in the relation between his job and my job.

First he left Ajax and, after three and a half years I left, too. It was difficult because it was a problem in the relation in football. But you know, if there is a problem in football relations then there is also a problem outside of football.

Never more was there the contact that we had before. Sometimes a little bit fighting [dispute] between each other.If there is a fight with two persons, both are involved in the problem. It’s OK. It is the past and we look forward. I hope he will be very successful.

And despite initially trying to play down the fiery relationship with each other, both managers managed to draw blood from the other during their first season in English football as Manchester United won the first encounter 2-1 on December 8, 2014, at Saint Mary’s whilst Southampton came out victors in the return tie at Old Trafford, winning 1-0 on January 11.

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The rivalry seemingly continued to boil on English soil and Koeman admitted that the pair failed to immerse themselves in the Premier League’s tradition of drinking a glass of wine together (via the Daily Mirror):

In England there is this great tradition where the managers drink a glass of wine together after the match. With Louis, that failed twice last season. At St. Mary’s he was lucky to win (2-1 last December). Still, the wine was ready, but Louis said he did not have the time to come in.

We went to Old Trafford and won (1-0, on January 8). Louis said to me: ‘Your glass of wine is waiting, I will come in a minute.’ I waited for a quarter of an hour, but apparently he was busy with the press. So I said, ‘Give Louis my regards!’

I started to walk to the team bus when Louis came out. ‘What about the wine?’ he said. ‘I have to go, Louis!’ I answered,  ‘Just like you that time at St Mary’s.’ I did not mean to be spiteful. I just said it how it was. Maybe we will have a glass of wine next season. At this very moment, I have nothing against Louis. We just shake hands and get on with coaching our teams.

Unfortunately, there seems to be no end to the spat between the former colleagues as the pair quickly picked up this season where they left off from last following United’s 3-2 win at Saint Mary’s on September 20, as van Gaal attacked his fellow opposite who disregarded his side’s poor defending ( via The Telegraph):

This is also remarkable that he says that. Have you seen the goal average against, also last season? All the media was writing the defence is very bad but at the end we were third in the Premier League. You say something and don’t think about what you are saying.

With all this in regard, this match is gearing up to be a fantastic spectacle and although Manchester United have not been the most attractive on the pitch recently, the fire can once again be seen in the bellies of van Gaal’s players following their 1-0 win against Liverpool last week. And if van Gaal can motivate his players to show no mercy against one of his bitter rivals then this game could well yet prove to be a classic – on or off the pitch.