Keisuke Honda is playing for CSKA Moscow in the Champions League tonight, so will be cup-tied if he joins Milan in January.
The Japan international is in the starting XI against holders Bayern Munich this evening.
If a player makes even one appearance for a club in the Champions League group phase, then he cannot play for any other side in the same competition that season.
Milan are believed to already have an agreement for Honda when his contract expires on December 31.
However, the creative midfielder has recently suggested they are “only one option,” as he also has offers from the Premier League.
It’s suggested the English clubs interested are Arsenal, Everton and Liverpool.