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.
Yes, it is not a must for goalkeepers, but goalkeepers who have the ability to do so have a plus point in this era of modern football. But again I say that it should not neglect the abilities of a goalkeeper that should be, as in the points I made earlier.
Now what happens to David de Gea is, when he is considered not to have good passing accuracy coupled with their defense that is not better organized. No matter how good a player is, he will not have a good pass when his teammates cannot open up space and can move without the ball.