Navas is an excellent goalkeeper, he was a key player that let PSG go far in CL in the previous season with his saves so counting on him for some games will remain even when Donnarumma will take his place step by step starting with CL imo or might be given the opportunity vs Clermont also.
Actually deciding between a bunch of players shouldn't be a problem tho, they will rotate them, the player that is ready more than the other will participate and so on, this might confuse further their opponents.
Don't get me wrong, I am not saying that Navas is a bad goalkeeper, I even said that in my post, he is definitely a good GK. However we are talking about something much bigger here, something about Donnaruma being much much better, this isn't about Navas being bad, this is about Donnaruma being that much better, dude could be one of the best GK of our era right now, and you are putting him in the bench for a GK that arguably worse but also factually so much older?
I would put Donnaruma in, dude left Milan for nothing because he wasn't happy there, if you keep putting him on bench here you think you get to keep him? The more you keep him on the bench the higher risk of him leaving as soon as his contract is over. They should be putting him on GK at all games except cup games and league games that are right after a UCL game which would make sense to rest him.
This would still give Navas around 15 games a season easily, whereas Donnaruma would be getting 20+ games easily as well, so it is not about if they should play or not, both should play but which games makes the most difference.
I didn't know that Argentina and Brazil had still World Cup qualifier matches right before the league matches. And Brazil - Argentina match was also postponed to another unknown date. This is still a very easy match for PSG even without Messi and Neymar both. I suppose Mbappe will carry the team again to a victory. 
