I think Galtier is having trouble making players run the system he wants them to run, that much is true and I would put majority of the issue on him. However, we need to also look at the team and how many players they put on bench and then put in. You don't sit kimpembe and hakimi down for example, what was the reason to do that at the start?
Plus, they still had some good players to beat Monaco, they weren't some "farm league team" neither, they were actually quite good on paper, so they should have been able to beat and won against Monaco and yet they failed. I hope for their sake they are not this bad against Bayern, they really need to play much better.
I think Christophe Galtier have personal reason why not including many main player in starting eleven when PSG beating by AS Monaco last match, PSG have busy schedule match in this week because several days later Bayern Munich opponent in first leg of Champion League. I think this reason from Christophe Galtier keep saving several main player in the bench and have preparing against Bayern Muncih.
But no doubt with his failure because can't manage well between main or reserve player, have bigger impact after PSG leaving by Mbappe seems difficult for winning on every match, before losing from AS Monaco in Ligue 1 have Marseille become nightmare for PSG in French Cup and have eliminated early.