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But you are assuming that De Gea wouldn't have made any mistakes, but in reality he has made mistakes more often in recent times. Onana's start for Manchester United was far from perfect, but Ten Hag also played games with De Gea and I don't know they parted ways, but I don't know if De Gea was still interested in staying. When Ten Hag decides against a player, he almost has ideological reasoning and doesn't allow a player to quickly get back into the game. They finally need a world class coach again and for me Ten Hag is definitely not one of those.
In short, De Gea has more experience in the Premier League than Andre Onana. This has certainly made De Gea better in terms of appearance, but a decline in performance due to age and tactical reasons has made Ten Hag choose to bring in Andre Onana to replace him. Andre Onana is not bad, but he has not been very good in his adaptation process to the Premier League.
I agree that fans need to lower their expectations for Onana's appearance. He will be fine this season if he gets good support, but if he gets pressure then it certainly won't help him change. Ten Hag must take a good approach to improve the form of each of his players, this includes improving the dressing room atmosphere to be more harmonious.