I’m not watching full games of them to be able to properly judge them, but the biggest flaw for me was a not fit looking Iniesta (he lost or even failed to fight for two chances to break up PSGs possession before two of the goals), which cannot surprise since he was injured shortly before. The other factor was Mascherano missing the game. He seems to be more important to them then ever before.
Another problem: since Dani Alves left for Juventus, the right back position lacks someone who can give them stability. Montoyas injury really weakened them.