Andre Onana is a good goalkeeper, but he needs sufficient adaptation to improve his performance. His performance was good but he conceded a lot of goals, but I think he will be more adaptive from game to game. I'm aware that Onana isn't at his best yet, so I don't think people should have high expectations for him too soon.
Overall Manchester United is not in good form. Onana is bad, as are the defense, midfielders and strikers. If Manchester United really needs improvement, then the entire team must work hard to realize what the fans expect.
If you must critize Onana, then you must have to deal with Maguire, Dalot, Evans, Lindelof, Amrabat and Varane. A goalkeeper can't do much without the help of the defenders, the reason why De Gea was able to do this as because he is experienced and know that he don't have to depend on defenders and that's why he doesn't have much mistakes but Onana is used to support which to me is not bad but they are really affecting his stats in Manchester united.
If you look at Onana stats in the last Champions League and Seria A, you will agreed with Erik Ten that his decision to bring in Onana was a wise one, he is so good but have not been lucky star in Manchester United. If he want to remain relevant, he need crush the opportunities for Manchester United to concede goals. He will not only receive much accolades from the club but will help transform Manchester United into the good teams they ones were where back then.