What about if a player moves in the January transfer window?
A club may register a maximum of three new eligible players for the knockout stage. Such registration must be completed by midnight CET on 2 February 2016.
One player from the above quota of three can have been fielded in a group stage by another club in the current season, provided that the player has not been fielded:
In the same competition for another club; OR
For another club that is currently in the same competition (applies specifically to third-placed UEFA Champions League teams joining the UEFA Europa League)
Can a player play in UEFA competition for more than one club in the same season in case of a transfer?
As a rule, a player may not play in a UEFA club competition (UEFA Champions League and UEFA Europa League, excluding UEFA Super Cup) for more than one club in the course of the same season. Such a player is called cup-tied. However, a player can play for one club in the first, second and third qualifying round, and/or the play-offs, of the UEFA Champions League or UEFA Europa League, and then represent another club from the group stage of either competition.