De Gea was the farce that went on for too long. We all saw that in the fa cup final. In this modern era, you cannot have a keeper that is rooted to the goal line. You have to play on the edge of the D.
Onana can play out from the back. Ten Hag has managed him before. You will see an immediate change in formation, because, like City and Liverpool, TH can move the entire defence to the half way line, and then overload the opposition. Which is what he tried to implement at the start of last season, but after losing to Brighton and Brentford, had to abandon.
In essence, he was forced to play an entire season with the handbrake on, because the goalie doesn't come out of his box and cannot play out from the back. This also invites more pressure, because your team has to sit back and keep their shape. Sometimes making De Gea look good, because he was pulling off worldie saves, but if your team are defending from the half way line, he doesn't have to make any saves at all.
There will be an immediate improvement. Pep demonstrated this when he dropped Joe Hart. Klopp when he signed Alisson. If you know that your keeper is going to come out and sweep up any balls over the top, then you can play further up the pitch and dominate the opposition in their half. If they can't get out, they can't hurt you.