Onana is over rated because he has better passing skills than De Gea but in general, combining all skills of a goalkeeper, I think he is not a better goalkeeper than De Gea. Hopefully Ten Hag will not have to pay an expensive cost by his coach position because of his belief in Onana.
André Onana is overhyped goalkeeper simply because his passing are accurate unlike David De Gea who's terrible at giving passes to his teammates. Manchester United are tol focused on the passing more than the brilliant saves made by David De Gea, who have been in the club for years and definitely knows how everything are done. He's capable and have saved Manchester United countless times but I don't think André Onana can be able to fill in the shoes of David De Gea, even though he's given more years, he can not represent the team's to his fullest capacity.
Modern football requires a goalkeeper who can build attacks from the ground with his accurate passing. But what happened to Machester United is that they neglected the things that a goalkeeper should have, namely good reflexes and also a good reading of the direction of the ball, and that is now not shown by Onana.
I would largely agree and say the same thing about Manchester United, which is that they have made a mistake in discarding David de Gea, although his weakness is not having good passing, in my opinion it is a weakness that can be corrected.