Inter Milan president Massimo Moratti has apologised for the mistake which saw Diego Forlan included in the club's UEFA Champions League squad when he was not eligible.
Forlan turned out for Atletico Madrid in the qualifying rounds of the Europa League before making the move to Inter on transfer deadline day.
He was therefore ineligible to play for Inter in Europe until at least January, but the Italian club failed to realise and named him in their Champions League list for the first half of the season.
Teenage forward Luc Castaignos was the unlucky player to miss out and, even though Forlan will not be able to play in the competition for Inter, it is too late to replace him.
"I'm sorry mostly for Castaignos because otherwise he could have played," Moratti told the club's official website.
"The Forlan oversight is fairly serious and it's not something we can fix, we have to find a way of avoiding it in the future."
Inter bought Forlan as a replacement for Russia-bound Samuel Eto'o, presuming he would be available in the Champions League, but they will not now have the chance to change their European squad until after the end of the group stage.
2010 champions Inter have been drawn in Group B alongside CSKA Moscow, Lille and Trabzonspor.
Defender Cristian Chivu, meanwhile, has given Inter an injury scare on the eve of the new domestic season after hobbling out of training with an ankle problem.
The damage will be assessed but it seems unlikely the Romanian will be fit to face Palermo in Inter's Serie A opener on Sunday.