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.