I think PSG's defense is not weak, PSG is not focused on their defense. They changed their match strategy. And their main goal now is to attack and score a lot of goals. So they are not able to focus on their defense. Romas is playing like he is. Although Ramos is primarily a defender, he is rarely seen in defensive positions. He wants to score or assist. As a result, the team may score, but their defense is vulnerable. Galtier should be wary of the performance of his defenders right now. Otherwise, they will face a big loss in the Champions League.
Not focusing on defense is a weakness that must be corrected, no matter how hard they attack if they forget defense it will boomerang for them. See how they were almost humiliated by Troyes for often not focusing on defense. If they change their focus strategy to attack because of the quality of the opposing club below them that is a big mistake they should not do again. If what you say is true, then they seem to underestimate their opponent, and if they do it again be prepared for the worst that will happen to them.
When PSG conceded first in the 3rd minute, with a quick goal scored by striker Troyes.
PSG responded by increasing their attacking pressure. however, in the absence of Danilo Pereira, of course, plays an important role in their defense. Presnel Kimpembe, didn't contribute much and his performance was terrible. in the end, Galtier pulled him off in the 59th minute and replaced with Marquinhos.
I don't think SPG underestimates any team they are against. only, Galtier's rotation didn't go according to their plan.
However, I think PSG can turn things around by winning the game.
and one more thing, SPG will play a champions league match and they have the same points as Benfica. So, it's only natural that Galtier's attention is more on the more prestigious competition.