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.
Build up isn't a must for goalkeeper, it's a must for defensive midfielder. Manchester United don't have enough good defenders to use that strategy, even Bayern Munich that having strong defenders yet still struggle when they have a bad goalkeeper.
Ten Hag don't have a vision, he don't know which player is good and fit with Manchester United's players.