There does seem to be some element of truth in what
@ItsEssexRob is saying.
To elaborate a bit, while I do think Evans, Jones and Smalling are very capable players (particularly Jones - just think the world of him, bit held back by injuries and occasional mental lapses) none of them get a unanimous vote among United supporters like Stam, Vidic, Rio, Pallister or Bruce used to. And TBF since the 90s, up until now we've had atleast one them through the entirety of a season. That sense of assuredness knowing you have atleast one top top CB is now missing. And while I do hope the three will up their games and emerge as leaders, it's not a certainty. And uncertainty lends to poor defensive records.
There doesn't appear to be any organiser of the backline of sorts like we've grown accustomed to. Thing is even if there is an average player in central defence, his weaknesses can be somewhat masked by a more astute partner. Perfect examples are Cahill (initially) at Chelsea (Terry), Alex at PSG (Thiago), Barzagli at Juventus (Chiellini), Lescott at City couple of seasons ago (Kompany), Fonte or at Southampton whose consistency suddenly went up in the presence of Lovren and so forth.
Right now we do lack someone of that ilk considering our projected starters - Shaw, Evans, Jones/ Smalling, Rafael. A top notch defender who's also a vocal leader and organiser at the back. Evans has done well in the past, but it's always been alongside either Ferdinand or Vidic who took up the mantle while Jonny was their understudy. Can he now manage the weight of expectations in their absence and lead some of the younger players ? Maybe he does.. But what if he doesn't ?
With that in mind if we manage to sign someone like Benatia or Sokratis or Hummels, someone who can really organize the backline and coach some of the less experienced ones I for one will breathe a huge sigh of relief.