If we’re talking about De Gea’s performance, it wasn’t that bad. What I can say is that he really had terrible defense during his later years at Manchester. The defense Onana has right now is more solid than what De Gea had. However, bringing him back to Manchester is outrightly the wrong move. His reflexes, which used to be his advantage, are no longer as sharp as they used to be. So, really, what’s the point?
Yes, De Gea's performance wasn't very bad. He got difficulty to perform consistently because Man United had too bad defense. Most of the players in the defense line were bad quality players, they couldn't play as a solid team as well. With this condition, De Gea didn't have the chance to perform as expected. So, we couldn't blame De Gea at that time. The real problem was in Man United board, they were unable to choose proper defenders to build their team. Regarding Onana, his situation isn't so different with De Gea. I think we can't claim that he is a bad goalkeeper. He may already do it with his best, but he can't perform as in the past because there are too many complicated problems in Man United defense.