I'm not so sure about Milan to lose to Dortmund in San Siro to be honest. Because they have gained some momentum finally by the important win against PSG in San Siro. These 2 goals were the first ones Milan scored in the group as well. However I think these goals will motivate them to be better at finishing in the remaining matches.
I must admit that Dortmund have been the best team at defending here so far. They conceded only 2 goals which were both against PSG. But I don't know if they can show the same away performance against Milan as they had against Newcastle before.
As a reminder Newcastle wasted many goal opportunities in that match as well.
One thing that people ought not to forget so easily is football records and history as most times the record some of these Teams keeps also reflect in their current matches. Now AC milan has the second highest champions league trophy so far, and AC milan is one of the best and competitive team in Serie A, though Dortmund has also been in an impressive form both in the German Bundesliga and the champions league as well but since they still stand at a reliable position in their on group, i think AC milan will be the ones to be much anxious of winning Dortmund in other to boost their chances of qualifying for the round of 16. So AC Milan match with Borussia Dortmund is one of the toughest match of the champions league. So let's wait in anticipation of the outcome of the result in their next match.