Forlan is blocked from the competition after turning out of Atletico Madrid in the Europa League qualifiers.
"When I understood that I was on the market and negotiations had started with the Nerazzurri, I decided not to play in the Europa League against Guimaraes," Forlan told Spanish radio.
"They told me all I had to do was not step on to the field against the Portuguese side in the play-off to ensure I wouldn't compromise my chances of playing in the Champions League with Inter.
"The truth is, I didn't understand the situation. I did not know the rules properly and thought that would be enough. I was told it was OK."