AC Milan says the season has been "altered" by Cagliari's stadium saga and has warned it could decide to take action.
It was confirmed that Fiorentina's match at Cagliari on Saturday (Sunday AEDT) will be played behind closed doors because of ongoing safety issues at its stadium.
That is the fourth match Cagliari has been ordered to play without fans, although the Roma match was forfeited.
Milan was one of only seven teams to play in a fully open stadium, with three others playing in front of just season ticket holders. The Juventus game was played in a neutral stadium.
In an official statement Milan says "any reasonable person would consider these decisions incomprehensible because identical cases have been dealt with differently".
It adds that "Milan reserves the right to take further action in the relevant quarters."
AC Milan coach Clarence Seedorf revealed he consulted former boss Carlo Ancelotti ahead of his team's Champions League second leg against Atletico Madrid.