Dependency on a particular player has always led to the down fall of most teams because I'm the case where such player suffers injury or is unable to play for some other reasons, probably had a red card ,the team suffers till his return, it's true that there's always a particular player who's always the game changer for any team but they shouldn't depend on him.
This kind of thing often happens to any football team and of course all teams that experience it will always experience decline in playing performance and make it difficult for them to win every match they play.
For the UCL, there are several teams that are experiencing problems with losing their mainstay players and only playing with the available players, but there is transfer window that can be used to bring in several players.
I hope that when the round of 16 starts, there will be an interesting match where each team shows their best performance and competes fiercely.
PSG has got very similar situation with mbappe as they have some level of dependency on him even tho they try not to make it obvious but then it could be observed in their performance in his absence how they almost not win games or find it difficult getting enough goals to be able to win games with good margin, this season they have played a few match without him tho but then it was obvious he wasn't in such games as their performance showed as one without their star player been present. For sure if he leaves they are definitely going to suffer the after effect awhile.
Mbappe is the only best player that PSG has after the departure of several great players they have such as Messi and Neymar.
I not sure whether PSG will be able to play well in the round of 16 because of course if they only rely on Mbappe then PSG will definitely fail in the competition and this is big loss for PSG.
Their mistake was that when Neymar and Messi left without immediately taking advantage of the transfer window that was still open to bring in other great players at the start of last season, they would experience real difficulties when the competition started.