Rio Ferdinand believes that Manchester United made a mistake by selling defender Chris Smalling to Roma back in 2020. United’s centre backs ranks have been a revolving cast since Rio Ferdinand and Nemanja Vidic’s reign of terror came to an end. Money was spent on Eric Bailly and Victor Lindelof but neither became the first-choice defenders United wanted. Then came the huge deal for Harry Maguire, which has not proven to be value for money. United went down a different route by signing a proven winner in Raphael Varane – but that hasn’t worked out either. Lisandro Martinez is a quality player b…