Manchester United tried to hire many players in the last summer when Ten Hag gave them the list of the players they wanted but still Manchester United couldn't have the good performance they were expecting to see from them. Manchester United started hiring more players this can be expensive for this team but since Ten Hag already hired fewer players in the last season, I think this time Manchester United will be more careful in hiring players to have better performance in the next season.
He didn't make adjustments and didn't recruit the players needed so even though he recruited some players it didn't help Man United's squad become stronger and more solid. Erik Ten Hak should try to find the players needed not just recruit players because if he makes a mistake next season then his future at Man United will be very risky to be fired. The defense line should be the focus because Man United are so weak in that position when they play and it is easy for opponents to score a goal their goal.
Now Man United have overhauled the management and coaching staff and they have managed to bring in some players now. This season will be a gamble for Erik Ten Hak because the management has made many changes as he wanted. If it still fails then I can say Erik Ten Hak will not last until the end of the season.
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.