Players are no longer cup tied in the CL after a transfer window, so not registering him for CL doesn't have anything to do with his potential transfer in Jan
In association football, a player who has appeared for a football club during a knockout cup but subsequently transfers to another club is ineligible to play for the new club in the remainder of that season's cup competition. Such a player is said to be cup-tied, i.e. tied to their original club for the duration of the cup tournament. They become eligible for their new club in the following season.
The rule is intended to prevent teams which progress in the competition buying talented players from teams which have already been eliminated, in an attempt to increase their chances of winning. It also discourages players whose chief priority is winning a trophy from requesting a transfer once their team has been eliminated from the competition. Since the introduction of transfer windows, which the cup-tied rule pre-dates, some have criticised the rule as outdated. Nevertheless, it remains widely applied.
Almost all cup competitions worldwide operate a cup-tied rule, but leagues do not (as leagues do not eliminate teams during the season). Cup-tied players are only prevented from playing in that specific competition, so for example a player who is cup-tied in the FA Cup may still be eligible to play in the League Cup (or vice versa). UEFA competitions are an exception: because teams can switch between the UEFA Champions League and UEFA Europa League[note 1] during the season, UEFA has a more complex system for determining whether a player is cup-tied in one or both of those competitions.
UEFA operates European club football competitions, primarily the UEFA Champions League and UEFA Europa League.[note 1] UEFA's regulations state that, with a few exceptions, players who play in any European club competition are subsequently cup-tied with respect to all European football for the remainder of the season.[1] On 27 March 2018, UEFA announced that it would phase out the cup-tied rule for the Champions League and Europa League, starting in the 2018–2019 season.[2]
The main current exception is the UEFA Super Cup, contested by the winners of the previous seasons' Champions League and Europa League. As this is effectively a prestige friendly with only one round, the cup-tied rule is seen as unnecessary. Representing a club in this fixture does not affect a player's eligibility in other UEFA competitions.[citation needed] Similarly, cup-tying did not apply to matches in the now-defunct pre-season UEFA Intertoto Cup, up to the semi-final round.
A further exemption applies:
one player per club who would normally be cup-tied can be registered and eligible to play, so long as his previous club did not field him in the same competition. This means that players can represent two clubs in the Europa League and Champions League, but only a maximum of one player per club. However, if the first club switches into the same competition (e.g. transfers from the Champions League to the Europa League through finishing third in the group stage), the transferred player becomes ineligible. This can lead to some complex situations.
https://en.wikipedia.org/wiki/Cup-tied#:~:text=This%20means%20that%20players%20can,of%20one%20player%20per%20club.
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I remember this cup tied situation has been scrapped in UEFA competition