Arsenal boss Arsene Wenger says the European cup-tied rule doesn't make sense after Pierre-Emerick Aubameyang was barred from playing in the Europa League for the Gunners this season. That's because the striker played for Borussia Dortmund in the Champions League, and BVB are now competing in the same competition as Arsenal.
The cup-tied rule is enforced in virtually every cup competition in the world. It was designed to prevent wealthy teams signing up good players from eliminated clubs to boost their own chances of success in later rounds.
Despite the transfer windows, first introduced in 2002-03, the principle still applies, as shown by Wenger's claim that transfer fees of "£50m, £60m, £70m in the middle of the season" justifies getting rid of the rule. Yes, for the wealthy clubs.
Wenger says "unanimously, everywhere in Europe now, the clubs are against this rule," but that doesn't mean they should get their wish.
It seems ludicrous to suggest a player can play for one Champions League club in the group stage, and another in the knockouts. That player could actually score for AND against the same team in one season.
UEFA has had many nips and tucks to its own cup-tied rule. It has evolved over time, and where it sits now works very well within the framing of transfer windows.
Original rule
Playing in Europe in any competition -- including the UEFA Super Cup -- would rule you out for any other club, in any European competition, for that entire season. The only exception was if a player had played in the three rounds of the Intertoto Cup (until its abolition in 2008), then they could be registered for another team. This was during the pre-transfer window era when players could move clubs freely for the majority of a European season.
2005-06
UEFA first tweaked the rule three years after FIFA introduced official transfer windows. Transfer windows had created two, clear periods for the movement of players, perfectly positioned with a European club competition season.
Clubs were allowed to register one player (of three permitted) after January provided they were in a UEFA competition "other than the one in which his former club is participating or has participated in the current season." So, basically, this would cover Aubameyang still being cup-tied as his new club (Arsenal) is participating in the same competition (Europa League) as his old club (Dortmund).
Tottenham fans will remember that Alan Hutton could not play in the UEFA Cup for them in the second half of 2007-08 after representing Rangers, who dropped down from the Champions League.
Some clubs have fallen foul of the rule when signing two players in January who had already played in Europe. In 2009, Real Madrid signed Klaas-Jan Huntelaar and Lassana Diarra, who had played for Ajax and Portsmouth respectively in the Europa League. Madrid had to pick one player and chose Diarra, leaving Huntelaar only able to play domestic football.
2006-07
The UEFA Super Cup was no longer considered for cup-tied purposes.
2010-11
The first major change for four years saw UEFA allow a player who had played in the qualifying rounds to play for a second team from the group stage onwards. However, a distinct condition was the player's original club must have not qualified for either group stage.
2012-13
UEFA made an even more significant change to the qualifying rule, meaning for the first time no player could be cup-tied before the start of the group stage. It was important for clubs who had players who were attracting interest in the transfer market, who felt they could not field their best players in qualifiers for fear of it leaving them cup-tied for the season, thus lowering their value or possibly scuppering a deal altogether.
Clubs had been able to make three changes to their European squads for the second half of the season, of which only one could have played in Europe before (as introduced in 2005-06). This rule was also modified to allow another one of those three players to have played only in the qualifying rounds of either competition.
Further changes?
The only realistic change left is the "Aubameyang rule."
There is no prospect of the cup-tied rule being abolished completely, as Wenger suggests. That could see Champions League clubs weakening eliminated UCL clubs (domestically at least) in January by signing three players just for the latter stages of European competition. It's not a healthy situation for the transfer market, and does raise the possibility of bigger clubs stockpiling even more players for the sake of a few months.
And the fact is that Aubameyang has played a part in Dortmund getting to this stage of European competition, even if that is by failure.
Think of it this way: a player is only allowed to play in the European path of one club within a season. By playing for Arsenal, in the same competition as Dortmund, he can influence the performance of two.