I totally agree with you that PSG doesn't look like they are going to maintain that form they used against big clubs last season in the UEFA Champions League because if you access PSG's last three matches, you will notice PSG's form is slowly declining, and they need to regain their last season's form if they want to compete in the UEFA Champions League this season. PSG struggled to beat Nantes in their opening French Ligue 1 match, but it will not stop PSG from winning the French Ligue 1 title because PSG has no competitors in their domestic league.
I also noticed that PSG’s performance has declined since their defeat to Chelsea in the CWC final. Their recent matches have been less than convincing, and even beating a team like Nantes proved somewhat difficult for them... despite dominating possession and having far better players, they still failed to collect many points from those games.
Although it’s still too early to draw firm conclusions, but compared to last season their current form seems lacking, and they may struggle against other European teams, especially those from the Premier League.