As for Onana, I believe he is a solid keeper but he is under a lot of pressure right now to deliver and the more mistakes he makes, the more pressure he gets and the more he fumbles. It's like a downward spiral for him right now and except Ten hag does something to fix his defensive team including Onana, they'll continue to suffer from defensive mistakes and errors.
Onana does not have good saving skills like David De Gea and his matches for Manchester United in Premier League show that statistics. In a match with Bayern Munich, his saving skills are shown again in the first conceded goal. He should save that shot better than making a goal for Bayern Munich. If the goalkeeper is De Gea, Bayern Munich shot would be blocked safely.
Onana helps Manchester United to passing balls from their defense area better than David De Gea but a most important skill for a goalkeeper should be saving.
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.