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.