By the time Lassana Diarra played his last game as a professional footballer, on 20 October 2018, he had become a “what if?” player. What if he had stayed more than a single season at Arsenal? What if he hadn’t made the catastrophic decision to leave Real Madrid and La Liga for Anzhi Makhachkala and the Russian league? What if he hadn’t made the even more disastrous move from Anzhi to Lokomotiv Moscow? What if he hadn’t had to pull out of Didier Deschamps’s Euro 2016 squad at the very last moment because of a knee problem?

Another injury forced his retirement shortly after a rare cameo for his last club, Paris Saint-Germain, for whom he was no more than a squad player. Diarra was set to remain a footnote in the history of some prestigious clubs, a series of unanswered questions, something of an enigma – but then we have a legal case, which will finally be settled on 4 October, putting the final full stop to a story that has been dragging on for a decade. It is Diarra v Fifa and if the player wins it could completely change the transfer market, possibly leading to anarchy.

There is a chance that this case is how the double FA Cup winner, French and Spanish champion and 34 times-capped French international will be remembered and how he will leave his mark on football, as the man who brought down the transfer market system as we know it. His lawyer Jean-Louis Dupont thinks so. Yes, that Jean-Louis Dupont, the man who also represented Jean-Marc Bosman in his landmark court victory over Uefa nearly 30 years ago.

It all started in the summer of 2014. Things had been simmering for a while between Diarra and his Lokomotiv manager, Leonid Kuchuk. The Frenchman had made an excellent start to what would be his only season with the Russian club but faded after the winter break and found himself on the periphery of the first team in the new year.

Lokomotiv believed that the drop in his performances justified a drop in his salary, which the player did not accept. The relationship deteriorated to the extent that Diarra missed training on false pretences – according to the club – on a number of occasions, prompting Lokomotiv to terminate his contract (which still had three years to run) in August 2014 and sue him for breach of contract.

The Russian club took the matter to Fifa’s dispute resolution chamber, which sided with them and imposed a ban on the player. Lokomotiv also sought financial compensation from Diarra, to the tune of €20m, the amount of the transfer fee they had paid Anzhi for his registration. The matter was referred by Diarra to the court of arbitration for sport, which upheld the Fifa ban on appeal and ordered Diarra to pay Lokomotiv €10.5m plus interest. “I will accept the situation as I have always done in the past,” Diarra said at the time. Well, not quite.

Diarra had not wanted for suitors after Lokomotiv had unilaterally terminated his contract. The Belgian club Royal Charleroi made him an offer, subject to a guarantee from Fifa and the Belgian football association that they would not have to pay the compensation sought by the Russian club; but when no such guarantee was given, Charleroi pulled out of the deal. This, in the eyes of Diarra, Dupont’s legal team, the French players’ union, the UNFP, and its international equivalent, Fifpro, who are named as co-plaintiffs in the lawsuit, amounted to a restriction of trade and a breach of European labour law.

The player had been prevented from engaging in his profession, as Fifa had refused to issue the international transfer certificate (ITC) which the engaging club, in this case Charleroi, needed to register him with the Belgian FA. Diarra’s argument is that he played no role whatsoever in the transfer discussions which took place between Lokomotiv and Anzhi, but still found himself blocked from resuming his career when he was denied his ITC by Fifa. The question now for the court of justice of the European Union (CJEU) is to decide whether Fifa’s refusal to issue the ITC was lawful or not.

As the ITC regulations constitute the very foundation of how the international transfer system works today, should the CJEU find in favour of Diarra on Friday, Fifa would have to overhaul the process which underpins all of it; and, right now, it looks as if it could be the case.

For this to happen, all the CJEU has to do is to follow the opinion issued last April by its own advocate general, Maciej Szpunar, who said: “There can be little doubt as to the restrictive nature of Fifa’s regulation on the status and transfer of players. By their very nature, the contested provisions limit the possibility for players to switch clubs … The contested provisions … necessarily affect competition between clubs on the market for the acquisition of professional players”.

Szpunar added: “The consequences of a player terminating a contract without just cause are so draconian that it is highly unlikely that a player will go down this route. The contested provisions are designed in such a way as to have a deterrent effect and send a chill down each player’s spine.”

The CJEU is not obliged to base its decision on its advocate general’s opinion, but neither can it ignore it altogether. Legal sources believe Fifa could have made a stronger case for itself when outlining the rationale of its transfer system regulations to the advocate general, and may be able to do so at this hearing. As other recent CJEU decisions have shown, notably in the Uefa-Super League case, European law allows for sports governing bodies to make regulatory interventions which restrict competition and would be considered unlawful outside the sports sphere.

The key is whether those governing bodies can provide the evidence that they are not doing so to pursue their own interests rather than those of sport as a whole. Should Fifa fail to demonstrate this in court next week, Dupont may have reason to claim that we are about to enter the age of Bosman 2.0.

“The likely practical outcome of Diarra will be that the transfer system in football, as we know it, will fall,” was one of the conclusions drawn by the Belgian sports law experts and academics Robby Houben, Oliver Budzinski and Melchior Wathelet – himself a former advocate general of the CJEU – when they examined the case in June.

What would replace it in actual terms is not clear, however, apart from one thing: Fifa would probably lose its overall authority over the transfer market, with collective bargaining involving clubs and players becoming the norm, as it already is in other sports.

How a fluid transition from one system to the next could be effected without creating a regulatory vacuum in world football – and cause chaos in the transfer market – is hard to fathom at this stage, which explains why the prospect of the 4 October decision is seen with such trepidation in some quarters – and nowhere more so than at Fifa.