True, Arteta may have underestimated PSG’s momentum. Beating Real Madrid is huge, but every round is a new battle. PSG showed sharp discipline and intensity—Arsenal can't afford another slow start in Paris, or their UCL run ends right there.
This match has nothing to do with the Real Madrid matches. Arsenal didn't play badly, they just could have been better. The early goal threw them off balance a bit. They recovered quickly, but we have seen this season how well PSG defend. Arteta maybe could have made the changes earlier and taken more risks, but he didn't go that way. He could have brought on Sterling and Zinchenko. He knew that if they conceded the 2nd goal things would be more difficult. He will take the necessary risks in Paris.