Yes, I agree that we should not blame only one player in this case, because now those who have poor performance are not only Onana, or it can be said that most Manchester United players make the same mistakes as Onana. It's just that Onana is more highlighted because he is a goalkeeper, and usually the slightest mistake made will be very visible, because one mistake will be punished with a goal.
It's different when a midfielder makes a mistake, then other players can still close, the defender who makes a mistake, the goalkeeper can still close, the goalkeeper who makes a mistake, who will be able to close?
The mistakes of onana has severe repercussions and that's why they needed him not to be making such mistakes regardless of the excuses whatsoever, like you rightly said if other players make mistakes, it could be easily and quickly made up for but his will cot the team the whole match.
Ten hag should use This January to strengthen defense line as that looks like one of the biggest challenge his team is facing currently and this aswell is his opportunity and chance to correct it , Manchester is not financially constrained at the moment so I think he has the privilege to go scouting to get these issues solved and get the team back to their glory days if he must keep his job, because bi think if he fails now again he may me relived of his job.
But in this case Onana also still contributed to several matches so I don't think we can completely make as if Onana is the most useless player at Manchester United because after all what he did in several matches there were actually some pretty good moments but when a player makes a mistake then sometimes we only see his mistakes without seeing the good performance displayed.
For this winter it would indeed be great if Manchester Untied had some additional players especially in the defence area but with the financial situation that we know sometimes the Glazer Family is also quite stingy then honestly this will also be a little difficult.