Three matches have been played and Manchester United have been able to win just one match, and the only club that is a strong club in the Manchester United group is Bayern Munich. Manchester United lost to Bayern Munich and also to Galatasaray, but I wasn’t expecting United to lose to Galatasaray.
You admit that Bayern Munich is the only strong team in group A. If Man United failed to finish in the second place, it will be very shameful. Man United is lucky they are actually in the easy group. I can't imagine if Man United is in group F, they probably will end up the group stage in the last position.
Man United must win the remaining matches if they want to finish in second place. They must forget the 2 losses from Bayern Munich and Galatasaray. They must be confident that they have the ability to win the remaining matches. At least, they must win against Copenhagen and Galatasaray. Then, they can target to get a draw when they play against Bayern Munich in the last match. I expect Man United finish the group stage with 10 points.
Fans will blame him when he makes a mistake and will praise him when he is the hero of his team's victory. What happened to Onana at this time was also experienced by Harry Maguire, the only goal Manchester United got to get their first win was thanks to a goal created by Maguire. These two players were often sneered at by fans in previous matches because they were considered to be the culprits in Manchester United defeat.
It is normal, mate. Fans will always blame the players when they do mistakes, and will appreciate them when they play with amazing performance. I can understand if fans sometimes blamed Maguire, he often did annoying mistake such as scoring own goals or too easy lose focus on stopping the opponents' attackers. Meanwhile Onana did a blunder, so it helps the opponents' attackers scoring goals easier.