I think there was fundamental problems with Manchester United's defense the goalkeeper shouldn't bear the brunt of the criticism. It takes a team to succeed, and when the defensive line isn't playing well even the finest goalkeeper may have trouble. Although Onana arrival may offer a new viewpoint it's important to remember De Gea accomplishments as well. In order to get greater outcomes, the team ultimately needs to focus on their defensive togetherness. Blaming one player won't solve the more significant issues they have been dealing with for a while.
I also think that Man United is having problems that if left untreated, it will hinder their achievements. Ten Hag should know very well that their defense line needs serious attention but somehow he prefers to sign a midfielder rather than a potential defender. Man United did indeed get Jonny Evans but I don't think he is the defender that Man United really needs at the moment because he is old and his performance may not be better than Maguire. What Man United needs is a tough defender who is not just a backup for Martinez and Varane but someone who can compete for a defender position in the first team.