Florenzi suffered a knee injury against ATM, I can't find any information yet when Florenzi can return to play. However, it was confirmed, if he will be absence when Milan vs Atalanta, Florenzi also threatened to fail to defend the Italian national team against Spain for UEFA Nation League match. More players get injured, vs Atalanta will be hard for AC Milan. Even Atalanta not on their good perform, i think Milan need to be careful, because they can beat them, I wish Giroud on starting line up on this match, so Milan have more chance to win.
AC Milan had a good season in 2020 - 2021 and got a ticket to return to Champions League. So far, in this season, it seems AC Milan don't have enough player resource to play in both Champions League and Serie A. I don't think it is bad for them in the first return to Champions League after many years.
They should concentrate on Serie A in order to get another ticket to Champions League next season. They also have to play good enough in Champions League to keep up the total points of Serie A compare to other national leagues.
The match with Atalanta will be very exciting because Atalanta is not far behind AC Milan. Atalanta have reasons to fight and complete this season in third or fourth position and be recognized as a new big club in Italy.