If I remember correctly, De gea has his fair share of mistakes that were costly to his team on multiple occasions. I still maintain my opinion that since he arrived at Man Utd, he is under a lot of pressure to be better than De gea and his confidence drops on each passing games especially if Man Utd defense as a whole can't really coordinate much. I'm of the opinion that Onana will do well at man utd as time goes and he rebuilds his confidence.
If you watched the UCL final last year vs Inter, you'd understand. But still too early. Let's see what becomes of him in coming games.
I believe if De Gea was making mistakes and conceding a lot of goals, and if Onana is facing the same thing right away as soon as he arrived, maybe United should realize the problem could be somewhere else? I agree with you that defense is lacking and its not good at all, but also I believe wings and midfield is lackluster as well. The best way to not have all that many shots to your goal would be making sure that you keep the ball, when you barely have the possession all game long and when your midfield is past easily with couple passes, then even a good defense can handle only so much. They do not have a good team all around, not just onana or defense.