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.
With more pressure mounting on him to protect the goal at all costs, more mistakes are bound to happen and the sooner Ten Hag fixes these problems, they’ll have to deal with loosing more games.
Onana has owned up to his mistakes in this match and hopefully, he does learn from it.
At the age of Onana, his skills are already solid and hardly to be changed or improved too much. Like we can not ask De Gea to improve his passing skills too much, so same for Onanan with saving skills. Ten Hag and Manchester United already chose Onana because they need a goalkeeper with good passing skills. In return they have to exchange it with a bit worse of saving skills.