It could be that they may not win the champions league again because is going to be hard to win the title and still go back to defend but they still have to try to get close to the semis or quarter finals like you said, you know they can't do it or get that far if they still have injury players who we all know are the reason or are the same guys that played a major role in them getting to where they are now as champions of Europe.
About the players I feel they can regroup and get back to join the squad, they will be out for 4 to 6 weeks respectively and I still see their replacement as players to cover for them till they get back. Enrique need to change his plans to the players to suit in if not they might be playing something different while the coach is instructing different as well but they are still going to make their opponents feel the pressure they are coming with just like they did with Lens in today's encounter.
If Dembele and Desire Doue return sooner and are able to play well, PSG chances of retaining the trophy are still quite high. Those two players will likely only miss three or four League Phase matches, and after that, Luis Enrique will have his ideal squad back. Actually, Luis Enrique doesn't lack rotation options for the front line, it's just that their quality isn't the same as Dembele and Desire Doue.
So far, Luis Enrique doesn't seem worried about the pressure in Ligue 1, PSG will remain consistent even when relying on second string players. Everything is clear on the league table, PSG has secured full points in every match. PSG will start their Champions League journey by facing Atalanta, which I think will be an easy match to win.