from wiki
"as per Champions League regulation 17.18, 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, meaning that this player can represent two different clubs in the UEFA Cup and Champions League. However, if the first club fielded him in the Champions League, but would later play in the UEFA Cup through finishing third in the group stage, the transferred player is ineligible to play in the UEFA Cup"
so they say that because atletico havent entered the comp he is elegible to play.