The Uruguayan striker will miss games against CSKA Moscow, Lille and Trabzonspor after featuring during Atletico Madrid's Europa League qualifying campaign
The Serie A giants attempted to include the Uruguayan striker in their 25-man squad for Europe's elite competition, but discovered that Forlan is ineligible after playing for former club, Atletico Madrid in their Europa League third qualifying round tie against Stromsgodset.
Forlan will be eligible to be registered for the competition if the side qualify from the group stage to the round of 16 which is due to take place in February 2012.
With the 32-year-old due to be absent for the group stages of the Champions League, Gian Piero Gasperini will have to rely on a strikeforce of Giampaolo Pazzini, Diego Milito, Luc Castaignos and Mauro Zarate
Forlan officially joined the 2011 Coppa Italia winners on transfer deadline day for an undisclosed figure after spending four seasons at Atletico.