Here is what it at stake:

1. WHEN WILL THE CJEU DECISION BE ANNOUNCED?

The Diarra v FIFA case features on a list of CJEU decisions, which are usually made public in Luxembourg from 0730-0830 GMT (0930-1030 local time).

2. WHAT IS THE FIFA RULE IN QUESTION?

FIFA’s Regulations on the Status and Transfer of Players (RSTP) says a player who terminates a contract before its term "without just cause" is liable to pay compensation to the club, and where the player joins a new club they will be joint and severally liable for payment of compensation.

3. HOW DID IT END UP IN A EUROPEAN COURT?

In 2014, Frenchman Lassana Diarra left Lokomotiv Moscow one year into a four-year deal and the club took the matter to FIFA's Dispute Resolution Chamber (DRC), arguing he had breached the rules when his contract was terminated after the player decided to leave without just cause following a pay cut.

Diarra received an offer to join Belgian club Charleroi, who backed out after FIFA refused to sign the International Transfer Certificate (ITC), preventing the player being registered with the Belgian federation.

In 2015, FIFA ordered Diarra to pay 10 million euros ($11.05 million) in damages to Lokomotiv, prompting the former Chelsea, Arsenal and Real Madrid player to sue the world governing body and Belgian FA before a local court.

Eventually, the Belgian justice system sought guidance from the CJEU, asking if FIFA's rules were in line with articles 45 (freedom of movement of workers) and 101 (prohibition of cartels) of the Treaty on the Functioning of the European Union (TFEU).

4. WHAT IS THE CJEU RULING ON EXACTLY?

The CJEU will say whether it is unlawful for FIFA to rule that if a player breaches their contract, which is then terminated by the club, their new team is jointly liable with the player to pay compensation to their old one.

CJEU Advocate General Maciej Szpunar has recommended that the judges side with the player.

"Some FIFA rules on transfer of players may prove to be contrary to EU law. These rules are of restrictive nature and may only be justified in specific circumstances," he wrote in a non-binding opinion.

5. WHAT ARE THE CONSEQUENCES IF THE CJEU RULES IN FAVOUR OF DIARRA?

FIFA would need to amend the article that makes the new club liable for compensation if a player's contract with their previous club is terminated. Although the Premier League, the biggest source of transfer funds in the world, is outside the EU it would be affected by any regulation changes as it falls under FIFA's jurisdiction.

Diarra's lawyers said this year that a judgment backing the player would be a milestone in modernising football governance in the bloc as it would allow players' unions and clubs to regulate their employment practices.

"This will put an end to the degrading practice of commoditising players," lawyers Jean-Louis Dupont and Martin Hissel said in a joint statement in April.