This is pretty big. It doesn't really affect the top-tier clubs, as transfers are
generally well-managed, but it is potentially a massive win for players in smaller leagues/clubs. While many predicted ECJ to uphold in favour of Diarra, most (including me) did not expect ECJ to go as far as deciding that FIFA's procedure for managing these disputes, and the penalties against potential buying clubs, were also against EU law. This is because CAS has previously upheld the joint liability of buying clubs in transfer disputes eg. Mutu and Juventus.
The main obstacle for players is not FIFA's rules per sere, it's the procedure. The rules as they stand allow a player to terminate their contract unilaterally for 'just cause' - but
you have to prove 'just cause', in forced arbitration. The burden of proof is entirely on the player, and an already lengthy-process can be delayed further by unconscionable conduct of the employing club. The player cannot complete an international transfer without clearance, and can't be registered to a new club without one, so essentially FIFA are the final arbiters. The balance of power is therefore with the club not the player, as most players cannot risk 2+ years on ice waiting for arbitration to conclude, just to seal a transfer.
The Rules also penalise the prospective new club; in Diarra's case, Charleroi could not pick up Diarra on a free, because they would not be able to register him and they would be jointly liable for any transfer fee compensation ordered by FIFA. This is the really interesting development - ECJ have ruled that this is potentially illegal, and have referred this back to the Belgian court to determine in accordance with EU law.
This system has long been criticised by Fifpro as it disproportionately disadvantages less wealthy players taking contracts in less stable leagues, who can't simply break their contract and find a new club.
Should the Belgian courts confirrm ECJ's decision as expected, FIFA will be required to reform their ITC system and develop a faster, fairer process (or scrap it entirely). I expect there might even be pressure internally for different rules for UEFA/EU and the other federations, and I also expect that FIFA will drag their feet in implementing a new system.
However, should a new system come about, it would have huge implications. A quick arbitration process (say 4-6 week process) could mean that 'bomb squads' and managers banishing unwanted players will be a thing of the past, as players could seek release from their contracts within a transfer window. In turn, in combination with the Webster ruling, it would make long-term contracts (eg Clearlake's 8 year contracts) functionally useless. And if fewer restrictions lead to more player movement, then transfer prices will likely plateau or even collapse to prior levels.
In short, Lassana Diarra might well have f**ked Clearlake's entire strategy in the arse.