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.
I am pretty sure that Klopp will give some of his usual starting eleven some time to rest in the game vs Milan and i also don't really think that this has an impact for the next games to come, at least not an negative impact. There are so many games in the english league and they have 2 cup competitions on top so every possibility where you can give your players that have a lot of time on the field a rest is very welcome and needed.
I don't think though that Liverpool will play with a complete B-Squad because the also have a responsibility towards the other clubs of the group. Everyone of the other 3 teams can still reach the knock out stage of the Champions League. Therefore Liverpool can not just give the game away for free.