We shouldn't put all the blame on Onana. I agree it's his responsibility. Because, to be honest, MU's start was not favorable. Everyone is at fault for bringing the team down. Onana's responsibility may be greater than that of the players. If he makes a mistake, he must pay the price immediately.
If you watch Manchester United vs Galatasaray yesterday you'll see that only the penalty was truly Onana's fault. He was not decent but the mistake he made that led to the goal was the penalty. It was a wrong pass.
Other than that, none of the other goals was his fault. The first goal, instead of Dalot to defend well was busy accompanying the striker. Onana is not so bad that he'll be sorely responsible for Man United's downfall.
The defense and midfield of Manchester United is very average and until they work on that they'll continue to suffer. De Gea would have suffered the same fate if he was still at United. A lot needs to be done in Manchester United but they're hell-bent on blaming the keeper for everything.