Regulations of the UEFA Champions League
2013-2015 cycle
XI Player Eligibility Article 18 General provisions
article 18.07
"As a rule, a player may not play in a UEFA club competition (i.e. UEFA
Champions League and UEFA Europa League, but not UEFA Super Cup) for
more than one club in the course of the same season. Exceptionally, however,
a player who has been fielded in the first, second, third qualifying round or the
play-offs of the UEFA Champions League or UEFA Europa League is entitled
to play in the UEFA Champions League or UEFA Europa League for another
club as from the group stage. Moreover, as from the start of the round of 16, a
player can be registered in accordance with paragraphs 18.18 to 18.21 below.
A substitute player who has not been fielded is entitled to play for another club
competing in the UEFA Champions League or UEFA Europa League in the
course of the same season, provided that he is registered with the UEFA
administration in accordance with the present regulations."
=>this is new
Subsequent registration
article 18.19
"One player from the above quota of three who has been fielded in a UEFA
club competition group stage match for another club in the current season
may exceptionally be registered, provided that the player has not been fielded:
a) in the same competition for another club; or
b) for another club that is currently in the same competition."