If Arsenal prioritized the the Premier League Competition over that Champions League Competition last season, then, Mikel Arteta shouldn't have gone all out in trying to beat Bayern Munich. He did that and lost both the Champions League game and also the next fixture they had against Aston Villa. In my opinion, he didn't play his chess well towards winning the Premier League Competition. They played but wasn't clinical when finishing.
Liverpool and Ac Milan are big rivals in the Champions League Competition, if the recent players do not remember, then the fan would definitely remind them to take this game like the Champions League finals. Tougher than anyone would expect, I can say Liverpool would win or Ac Milan. Maybe Ac Milan!
Arsenal lose so many players: Zinchenko, Saka, Odegaard, Merino, Calafiori, and Tomiyasu, if Arsenal will play without them, I doubt they will able to beat Atalanta. These players are very important for Arsenal especially Saka, without him, Arsenal is rarely to score a goal.
As for Liverpool vs Milan, both of them have a chance, anything can happen. However I expect both of them won't score many goals.