Arne Slot fielded a second string squad in the final League Phase match against PSV, which resulted in their defeat. In my opinion, if Arne Slot had not rotated his squad, we might have seen Liverpool leading the standings with 24 points. Meanwhile, PSG had a very hard time reaching the round of 16, so it is natural that they are not the favorites when facing Liverpool. Of course, I don't mean to underestimate PSG, but on paper Liverpool are more worthy of being the favorites.
The second string squad were fielded in that fixture because it was obvious they had almost nothing to loose if they don’t gwt to win that game so it was just ideal rotational technique was applied in that fixture as it did helped in the future fixture after that. I'm believing that Liverpool is going to have their main starting 11 against PSG because a whole lot is at stake in that particular fixture if Liverpool should loose it.
Every team will put their best players on the field. That's why I think they will play with a rotation squad in the matches they play in their own league. This situation will be less challenging for Paris Saint-Germain than Liverpool because the French league is not a very challenging league like the Premier League. That's why Paris Saint-Germain will be able to rotate as they wish.