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.
Saka and Calafiori are questionable, so there is a chance that they will be able to recover for the match with Atalanta. And Arsenal plays much more consistently, their game is not as good but still it is better than the ups and downs that Atalanta has, like losing 4-0 to Inter. These teams are focused on attacking football, so you can expect a lot of goals, but I remember how Arsenal played in the Champions League, it seemed like it was a completely different team, Arsenal in the Premier League in the Champions League stood out for its scoring.
Atalanta last game against fiorentina, atalanta won 3-2 with their talisman Ademola lookman firing from all cylinders. The game against Arsenal will certainly be a tough one because atalanta will be playing with the mindset of meeting big side, so they will come out all in. The Arsenal of last season's champions League will be a bit different from this one, I believe they will be playing with an experience mindset and with a winning mentality driven from their last win to Tottenham. I believe is one of the interesting games we will be watching in the champions league as both teams are very unpredictable most times with their attack, though arsenal are the big boys here, but let's just see what Atalanta will do on Thursday since they are hosting Arsenal, maybe some home advantage with creep in.