What Ten Hag is currently doing should be like what you said. Why is he so insistent on getting a new defender because he knows that is Man United's biggest weakness, not just last season, but it has become a scourge for Man United in recent times. To even make it real, he made 3 offers and had already gotten one of them. Leny Yoro could be a good partner for Lisandro Martinez but, there is a possibility that Man United will also make De Ligt's transfer happen.
Meanwhile, another weakness is in the defensive midfield line. Casemiro and Amrabat were expected, but apparently they both not to be so impressive and for this reason, Man United is making an approach to Manuel Ugarte. I think it's only a matter of time until Ugarte comes to Old Trafford.
The three names will be the players needed by Erik Ten Hak and he knows the weaknesses last season when recruiting several players so this season he is starting to try to fix them. If De Ligt is successfully brought to Man United then their strength will be much stronger and Erik Ten Hak only needs to fix the existing players so that they can play more consistently next season. Defenders are a problem for Man United because some of the players they have are often injured so they have to realize the transfer this season.
The midfield is still a little better because Man United has several players who can be utilized and young players have also developed in that position. I see Man United's weakness in the defenders and that must really be fixed by Erik Ten Hak because it is so easy for opponents to break into their goal.