Onana is good but De gea is classic with a lot of experience than Onana. Where De gea was lacking was in having the confidence to create passes with his players stead of always shooting the ball away back to opponent ground dispossessing his team and that's not the kind of goalkeeping style Ten Hag wants that's why Onana has to replace De gea from Ajax.
I think Onana needs to be giving more time to work on his mistakes and that's what ten hag is doing, only that as time goes on his mistakes is causing the team some drawbacks with match results, even in those matches they could win they have to fight for a draw. because Onana couldn't save but concede a shot that most goalkeepers could have to saved.
But Onana is a ball playing goalie and he's very confidence in receiving ball from the defenders calm the situation and distribute it to the attackers, when it comes to playing from the build up you can count on him and that's what Erik Ten Hag likes because it's suitable for his pattern, but when it comes to shot stopping he's not really bad, but he's no where compared to David de Gea cause he loses focus atimes I think Onana is being pressured by the fans and he needs to be calm and work on his shot stopping.
While the did has already been done, Manchester United need to adapt to the system as it is by preventing blunder from occuring at the defense and making sure to score more goals than their opponent in every match so they don't get drawbacks from Onana's goals conceding in each game,