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.
Mikel Arteta never underestimated PSG in any manner in that game, Arteta played his best first eleven which came all out to beat PSG but they met a better and stronger PSG team than they had envisaged. PSG eliminating all English teams Manchester city, Liverpool and Aston villa in the UCL gave me that understanding that Arsenal wouldn't be any different despite how they had to beat Madrid home and away. Maybe, Arsenal can go to Paris in the second leg and upturn the result to their favour but that doesn't seem easy because PSG chances of also scoring in their ground is very high.