It is true that if Mbappe leaves PSG on the days when PSG is pursuing every competition that is followed, it will be very difficult for PSG, because in my personal opinion PSG also does not have a replacement for Mbappe to perform well at this PSG club. But this is still just a rumor, we can't be sure what the future will be like after all these rumors happen, whether mbappe will continue to stay at PSG or indeed leave in the January transfer market later, and basically also that real Madrid does need mbappe to strengthen its squad if you think about it, but we are really confused by mbappe's behavior which always gives a controversial attitude.
Now Mbappe will definitely not be able to leave PSG because he has a contract until the end of the season, but he can already start negotiating with other clubs. I read the latest news today that Liverpool wants to get Mbappe, I don’t know if this is true (as the French publication Le Parisien claims), but it seems that Real is still the number one contender. I don't think PSG will try to stop him, they will get big money for Mbappe and it looks like they are already looking for his replacement.
Mbappè can't leave in January if PSG doesn't want to sell him, but if Madrid is able to buy Mbappè at the agreed price I think PSG will let him go rather than PSG having to lose Mbappè without getting any profit because Mbappè is PSG biggest investment at the moment so let it be this player being free is a painful thing, if in the end Mbappè leaves PSG, I don't know what will happen to the team because so far Mbappè has been a key player for PSG in achieving success, whereas currently PSG doesn't seem to have any rumors relating to the striker Mbappè replacement.