I think that fans can expect their goalkeepers to not make many of these absolutely unnecessary mistakes that destroy the whole game. When a player plays seven years for Ajax Amsterdam, then plays for one year for Inter Milan and then gets signed by one of his former coaches, I think that Ten Hag knew what he was doing and he took the risk to get rid of De Gea and if I recall this correctly he also didn't treat De Gea very well. Now they have to work with what they have as really good goalkeepers are quite rare. Onana has already had a few mistakes that were not really unacceptable, but maybe it is the time he needs to adapt although I am not really a big fan of blaming his lack in experience. He has been playin on a top level for quite some time and he isn't 18 years old anymore, but 27.
The arrival of Onana came with alot of expectations cause as Erik Ten Hag succeded in kicking David Degea our of the team alot of fans expected a very better and talented player that's young and would serve the team for a long term but however Erik Ten Hag decided to bring in one of his former goalie he trust and since then he's made alot of errors thats cost the team to lose matches.
Well if Onana want to maintain his role at Manchester United then he should train daily and try his best to save his team some embarrassement this season, and at 27 it would be very bad to be performing very bad for a team like Manchester, well the coach believes in him but I feel sooner or later if Onana fails to improve he'll be replaced with the 2nd choice keeper from Turkey.
Training hard on a daily basis is what a goalkeeper can do and maybe have some mental training as well. I remember when ter Stegen went to Barcelona, he also had some critical moments when his gameplay wasn't looking all that safe. But over time he improved and had lots of opportunities to prove his goalkeeping quality and he conquered the number one position at Barcelona eventually.
I think it is too early to talk about Onana's mistakes only and how they cost Manchester United points and games. I think there are also more players than just Onana playing far below their maximum potential. I really can't think of a player than Ten Hag made better. Although this is his job.