Onana is not very consistent, he often moves in situations he shouldn't. I think he should be able to control his instinct level which is quicker to catch the ball rather than continuing to make blunders.
However, the final result was in favor of the hosts where they were able to get 3 points at home which made them temporary leaders in Group A.
At some point I do say Manchester united miss de gea, because onnana seem so unstable and irregular, he exhibit some level of unnecessary confidence which I think at some point is not needed and this has led to certain mistakes which cost the team the game, especially in this just concluded UCL game where he was not at his very best.
Onana should understand that he is a goal keeper and that's his job, he shouldn't see him self as a player, rather 90% goal keeper and 10% player. He should hold the ball and stop trying to use his leg all the time as this could lead to possible mistake which the team may not be ready to tolerate as when it happens. I hope he becomes more careful and perform better.
The defense mechanism of Manchester united don't seem formidable enough at some point, as the give too many spaces for potential penetration by their opponent, as displayed in the second half of the UCL game they played yesterday, sometimes a good defense system could be of great help to the keeper as the safety of the goal post does not solely depends on the goal keeper alone.