I think the time is right for this transfer and while many people did't see this coming, it was the right decision for him.
When you say that they shouldn't lose their crucial players, at the end of the day it will be equalized when they get another player for him. Money isn't a big deal for them and they will purchase a player as a substitute no matter what the price is for them.
The presence of Nuno Mendes is expected to be an additional strength for PSG in facing the competition in the Champions League because Luis Enrique needs a player who has good performance.
Losing key players or new recruits in football is something that is normal because the transfer market is opened for such needs, but how can the club make a balance when one of their key players leaves.
PSG have lost several star players in the team but they are also looking for replacement players.
Now the big target that they are preparing is to meet the expectations in the Champions League competition and this is the target they want to achieve.
Yes but single players may not significantly raise PSG's chances to ever win the Champions League. If you look back, at one point they had Neymar Messi and Mbappe and all of them were in great shape and yet they didn't have a chance to win the Champions League because there was always this one game when the whole team messed it up because they couldn't play their game the whole distance and started making mistakes while having full control over the game.
Enrique will of course keep trying, which is the least he can do. But of course I wonder what set of players they should try to sign if Messi, Neymar and Mbappe wasn't enough to win the Champions League.
Nuno Mendes is a great addition beyond doubt and being ambitious about future goals is important, but having too high expectations doesn't make sense to me. They should go game for game and then see if an opportunity arises or not. But calling out the Champions League as the major goal doesn't suit them very well. It's different when Real Madrid does it because they won it more than a dozen times.