Manchester United did the right thing allowing David De Gea leave but did wrong signing a goalkeeper like Andrea Onana. I get it though, this was a signature wanted by Erik ten Hag during his second season with the club so as to execute and project his tactics and style of play. Disregard speculations of David returning to Manchester United, he has been offered contract extension already by Fiorentina ( 1 Yr extension ).
But I think it was a wrong way to let De Gea leaving. In Fiorentina, De Gea shows that he has a better quality than Onana. If De Gea still plays with Man United, Man United may have no problem with goalkeeper position. To be honest, I don't really knows Onana but I think Onan is actually not really a bad goalkeeper. He only can't be as strong as we expect because Man United has fragile defenders. There is no way that Onana can perform optimally if Man United never improves the quality of their defenders.
This has nothing to do with bad defending from Manchester united defenders, it's more of onana cutting out this constant blunder he keeps on doing constantly because aside that, he is actually a great goalkeeper, just that the pressure of his constant messing up at the goal post has made him to play without confidence, which is the major reason behind the blunders he usually makes.
Then as for de gea, it's very obvious that he is a better goalkeeper than onana can ever be because when he was at Manchester united, he hardly makes stupid decision that leads to goalkeeping blunders, and his saving ratio is way more better than that of onana, so I see it as a mistake on the part of Manchester united for letting David de gea leave the club when he did two to three years ago if am not mistaken.
But regardless of what so ever, onana really needs to cut out this constant blunder at the back if he truly want to stay at Manchester united for long, because I think that it's now getting to the point where fans are now calling for his replacement, because his overall performance and contribution at the club hasn't been good enough since he came to the club.