It cannot be denied that Mbappe's influence is quite big for PSG and if Mbappe leaves in the transfer this January then I am very sure that PSG's progress in the UCL will not be long.
The presence or absence of Mbappe I don't really put great hopes for PSG in the Champions League because if you pay attention when they have a trio of world attackers, they also cannot give anything. This is not about one person but how the coach can motivate other players and the condition of the coach is also quite a role to bring his team successful in any competition.
However even though Mbappe is still surviving doesn't mean he will be able to lift this team's performance alone, I think PSG is not worthy of becoming UCL champions this season because we have to look at the strength of the team as a whole and PSG does not deserve that, especially this season their performance is very good inconsistent.
PSG needs to consolidate a number of players and maybe Luis Enrique needs a lot of time to bring PSG to be one of the clubs calculated in the Champions League. Mbappe does have its own role in the squad but if you hope that this will not make PSG able to win the Champions League title. If it is true that Mbappe leaves, it is time for PSG to make a reshuffle to bring in other players and that's when they must set a target if they want to succeed in the Champions League.