Blaming Onana completely is actually also bad. I think there are many factors when Man United concede a lot this season. You have to remember this season there are not many Man United defenders in good shape. Many Man United players are injured. So it was purely in the team not Onana. Ten Hag's strategy also has a lot of blunders. The players made a lot of mistakes. The flow of the ball from below does not go well, possession is easily lost. When under tight pressure, Man United is always weak to protect the ball. Many players are confused about where the ball will be given. It's a classic problem but Ten Hag has to work hard to eliminate these mistakes.
Onana did remarkably well in the middle of the season; it was unjust to hold him responsible for everything bad that happened to the united. To be honest, United's lack of a strong defensive line last season was one of the main reasons for their poor performances, even breaking the record for the most conceded goals in United history.
And many of United's defenders injuries greatly affected the team's performance. Ten Hag made some incorrect decisions, but the injured players made him do so.
Any factor contributing to United's poor performance in the previous season makes it challenging to blame Onana alone.