I'm not sure if Ten Hag can solve the problem on Man United defense although he will buy new defenders in January transfer window. Ten Hag isn't a typical coach who can choose proper defenders. Many of the the defenders he bought, played underperformance. He also didn't want to sell Maguire even if he knows Maguire is one of the problem in Man United defense. I don't know what Ten Hag thinks to solve Man United problem. He seems have a different view than common managers. Anyway, I assume Man United management also need to evaluate their scouts. Man United scouts look not working very well in the last few years.
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.
And what is very surprising is that Maguire, who was planning to be sold in the transfer market, has now shown his best performance and is quite helpful for Manchester United's defense line.
all the players that Manchester United is currently recruiting are his wishes, but in fact the players he is recruiting do not fit into his strategic scheme, that is very, very strange.
The problem with the Manchester United team is not only in the defense line, but in all lines, so the team's poor play makes it very easy for opponents to launch attacks on the Manchester United team's defense line.
I don't know how Ten Hag can improve a team that has fallen so badly, and I'm pessimistic that he can do it.