Not sure about Milan who will play away against Bayer Leverkusen, although Bayer Leverkusen is not as good as last season but I personally believe Bayer Leverkusen is still able to get good results this season including in the Champion League. Thus, although I don't relate to the odds comparison, but I personally will still choose Bayer Leverkusen to win. Indeed, in the past Bayer Leverkusen was not a great team and did not even have a history in the Champions League, because if you consider about history then of course Milan is still better but at this moment they are different.
But indeed, Milan also managed to beat Inter in Serie A and it's like a sign that Milan is able to rise with its new coach. But I believe, the tension in the Champions League is certainly very different, moreover they are also still targeting victory or points to stay in the top eight. Thus, I think Bayer Leverkusen will still be able to stay strong and not lose against Milan.
On paper, victory seems to be on Leverkusen side, even though we know that AC Milan has more experience in the Champions League. Since being handle it by Paulo Fonseca, the Serie A representatives have not performed consistently in domestic competitions, they also lost to Liverpool in their first match. Leverkusen is now a team to be reckoned with after recording an impressive season, and currently Xabi Alonso's squad is also considered to be able to maintain its consistency in the Bundesliga and start their campaign perfectly in the Champions League. So it makes sense to assume that Leverkusen has a slightly higher chance of securing full points.
It is true that the intensity and atmosphere in the Champions League is very different from other competitions, where usually the more experienced team often looks more dominant. However, Leverkusen now has all the attributes to create luck and AC Milan really have to be wary of them. Most recently, AC Milan managed to win a landslide victory in Serie A, and we'll see what will happen to Leverkusen when they face Bayern Munchen.