Ten Hag's only option right now, in my opinion, to keep Manchester United's defense solid is to hope that the club doesn't fire him before January when they will need to acquire new defenders to cover their goalposts. Apparently, Onana needs a strong defense that would make it difficult for other teams to score him easily. He is not to accomplish it on his own without the help of Manchester United
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.
There is always solutions to every problem the defense have seen all of their good defenders put and injured the last game saw another injured and it affected how they played also. Ten Hag needs all the support you can give him to change the conditions of the club. The club and fans have past glory so they want to pressure until they see the club at the position they have always been in the past. Onana has been trying not enough but has given heart and mind for Man United .
Jim Radcliffe will support him with the basic necessary support to take Man United to the top. Some signings are not meant to play for united.