This is a very important match for AC Milan and Borussia Dortmund, AC Milan is a bit of a favorite because this match will be played at their home ground and also in my opinion if we look at the qualities of the two teams they are not much different, so both of them still have the same possibility of winning match.
AC Milan let some players have time to rest and prepare for this important match. With an advantage from home stadium, returns of some star players and their more experienced and DNA in Champions League, they are above Dortmund in theory. This match is important for both of them but at home, AC Milan will not let Dortmund leaving with three points. I believe AC Milan will be able to get at least a draw but it is not what they need from this match. They need a victory to have good advantage before the final day of this group.
Borussia Dortmund might be a little more comfortable because they only need a draw and even if Borussia Dortmund loses they still have a chance to qualify, but not for AC Milan in this match AC Milan will give everything because if they can't win then it's almost over. make sure they won't make it out of the group phase.
Dortmund will play more defensively in this match at San Siro and they will mainly wait for chances from counter attacking as they are well aware that if they lose this match, they will almost say goodbye to Champions League.