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As far as I remember, the meaning of the restrictions when changing clubs is so that a player who started a tournament in one club cannot play for another team during this tournament (probably to avoid the paradox when a player plays for team A in the group stage, but against team A in the playoffs). Mbappe has not played in the Europa League this season so there are no restrictions on him. By the way, it’s funny, but a paradox could theoretically arise. For example, if a player plays in the Champions League and his team is relegated to the Europa League, and the player himself changes clubs, then in the Europa League playoffs he can play against his former club.
Yes, I also think that these are nothing more than rumors and such a transfer will not take place in the winter.
I did a quick google search and found
the following excerpts from the rules:
"46.01: As of the round of 16, a club may register a maximum of three new eligible players for the remaining matches in the current competition. Such registration must be completed by 2 February 2023 (24.00CET) at the latest. This deadline cannot be extended.
46.02: Any or all of the players from the above quota of three may have been fielded for another club in the qualifying phase, play-offs or group stage of the UEFA Champions League, UEFA Europa League or UEFA Europa Conference League."
This means it is indeed allowed to register three new players that were signed during the winter transfer window regardless of whether they played for another club in the respective tournament already. The only rule is that not more than three players can be registered for those competitions. I think I remember that Chelsea ran into problems some years ago when they signed a lot of players and had to tell some of them they won't get registered for the Champions League.