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Rio Ferdinand, 33
Ex-England international Ferdinand is in the twilight of his career. He's been found wanting several times over the past few seasons and Gareth Bale exposing his utter lack of pace at Old Trafford this season was the final straw for many fans.
He's going to continue declining and while he is most certainly a serviceable backup, a long-term plan must be made at his expense.
Nemanja Vidic, 30
On his day, Serbian monster Vidic is the best central defender in the English Premier League. The problem is, "that day" hasn't come around for nearly one whole year.
At 30, with injuries creeping up, there are no guarantees on his future. He is still absolutely indispensable because if he returns to fitness he'll be a beast, but again Sir Alex Ferguson must plan ahead.
Chris Smalling, 22
Smalling should be pretty close to returning to full fitness now after his metatarsal injury. He's a very efficient player who divides opinion, but it's likely you'll see the former Fulham defender donning a Manchester United shirt for the next five to six years.
Phil Jones, 20
Jones will be fine when he stops being messed around with regard to his position—he's not a right-back, so don't play him there.
He seems most at home in a box-to-box midfield role like he played at Blackburn Rovers, but right now he's always one injury away from filling in at central defence.