Antonio Mirante, who is now at Milano, undergoing medical visits priot to signing the contract with the club, can be included in the UEFA list.
The former Roma goalkeeper is set to replace Mike Maignan who will have to go through a surgery on his left wrist.
Milan did deliver its UEFA list in September and the regulations don't allow to modify it now until February if the club reaches knockout stages. But, here's an article, as relayed by MN that allows an exception:
"if a club cannot include at least two goalkeepers in List A due to long injuries or illnesses (eg 30 days), a club can temporarily replace it in any moment during the season."
Therefore, Antonio Mirante will be part of AC Milan's squad list in the UEFA during the remaining games of Champions League group stage.