The next fixture is indeed a bit harder for Milan but I guess Liverpool will loosen up something to give some starting eleven players a rest. With the perfect points they got so far, Liverpool has the discretion to give their substitute players a chance to show up.
I hope they didn't, Liverpool is still on fire, when they loosen up the games it could be affected for another game in the premier league. I hope Klopp doesn't change the formation within the full team like usual, Salah still has a chance to create a new record in the champions league. Right now, Salah is the top score in the champions league but it is possible he will defeat Lewandowski with 8 goals. While Milan, only miracle to bring the team to 2nd round, only if Porto vs Atletico ties.
Milan can also move to the next round if they win their game and Atletico Madrid wins as well but they do it while maintaining the same goal difference, so if Milan were to win 1-0 and Atletico were to win 2-1 then the goal difference between the two will remain the same and Milan will move to the next round, still I do not find it likely they will in their next match against Liverpool, because even if we think they could play some of their players on the bench those players will play with everything they have to show they should have a chance to play with the team more often, and that could be more than enough for Milan to not win that game.