Not only strengthen the defenders, Onana also needs to improve his performance and avoid unnecessary mistakes. I think Onana also still didn't play consistently, he probably still adapts with Man United players and tactics. But he must adapt quickly because it will bring big impact for Man United if he fail to adapt well. He must be aware that he is Man United main goalkeeper now.
That's the importance of Onana's role. He must adapt really quickly to the players in the back line so he doesn't make unnecessary mistakes in communication with several Manchester United players. I think Manchester United will not make it through to the next round in the UCL. They will likely fail because Manchester United is really not ready to be in a big league like the UCL this season.
I think they can focus on the EPL this season, there is still a lot of time to improve their current position, at least they can be at the top to be able to prove that Manchester United can rise from adversity, that's why Onana should ensure that he is the main goalkeeper who has an obligation guard closely so that the goal is not easily conceded and avoid losing again.
This is no longer an adaptation problem for Onana, but the task that Erik Ten Hag gave him is very burdensome. I mean if we look at Onana arrival at Manchester United, Ten Hag puts more pressure on the goalkeeper attacks (build up) every match which is almost obvious and this tactic is completely ineffective. You must have seen when Build Up starts from Onana then passes to the defender - Onana - back to the defender while the opponent presses to reduce the distance, sometimes a passing error occurs and Onana throws it away. Such is Ten Hag strategy that he spends a lot of time not knowing when the front line players are needed to distract the opponent. There's nothing strange at first glance, but if you look closely, Erik Ten Hag Build Up doesn't catch the opponent off guard because they already know Ten Hag's cheap tactics.