Onana should have been the solution after Manchester United management kicked out David De Gea, the fact that is currently happening is that Onana doesn't seem to have finished his adaptation period in The Red Devils squad. The series of injuries suffered by most of United defensive players increased the number of goals conceded by Onana, in the end his move from Inter Milan did not seem like the right solution. Even though Onana is neither bad nor very good, he is the player who gets the most criticism, if United's performance does not experience a steady improvement in the future, it will be difficult for Onana to avoid criticism from fans.
With that defense and midfield that Manchester United has, any goalkeeper will concede a lot of goals. The defending is poor.
Manchester United concede too many shots on target. As a team you shouldn't let your opponent have that many shots, talk more about putting them on target.
The United fanbase all blamed Onana for the Galatasaray loss but that loss was not on Onana, it was due to poor defending.
The midfield doesn't keep hold of the ball much and they don't create enough chances, and the chances they create can't be finished by the attack. So you see that the whole team is not working.
United have not comfortably dominated any game, and that's a problem for the kind of football Ten Hag is trying to play. If they don't start dominating games they'll not be able to win games.