I think that the list of positions in need of strengthening has changed recently, due to the news regarding de Ligt. First and foremost, let us all hope that he gets back his health. Should he return to the squad and regain his earlier form, great, that will be a bonus. It does, however, leave a gaping hole at CB.
Martinez is not even able to manage half the games in a PL season - he averages just 17 apps in the PL during each of his four years at United - so for all his talent, he simply cannot be relied upon. Maguire thankfully signed up for another year, so at the least, a dependable and very experienced campaigner is in place. Heaven is really starting to look the part. The lad has a way to go but is developing really well. Yoro, less so but he remains an important member of the squad and should be going forward provided he learns about life in the PL. Whilst Shaw and Mazraoui are both capable of filling in at CB, both are again, unfortunately prone to injury. Fredricsen and the untested unit Godwill Kukonki are coming through the ranks but at best, perhaps one of those lads can fill in as the 4h / 5th choice CB. United need to sign someone and they need to be good.
CB has been added to the list of needs, along with the obvious CMs, LB and likely depth for CF and a LW option.