Forlan, who joined Inter at the trading deadline last week, played in Atletico’s Europa League third qualifying round matches against Norway’s Stromsgodset.
UEFA says its rules allow a player who appears in qualifying rounds of its competitions to transfer and represent a new club in the group stage, but only if his original club fails to progress.
The Uruguay forward is therefore cup-tied for Inter until the knockout rounds begin in February.
Inter is top-seeded in a Champions League group with CSKA Moscow, Lille and Trabzonspor.
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