In his column for the Telegraph, the former Reds captain explained: "In my opinion, Ferdinand has been the best defender in the history of the Premier League and, while his loss of pace means he is no longer a great player, he is still a very good one.
"Rio was once blessed with pace and, when he was at his best, was undoubtedly one of the best defenders in the world. But the injuries that have stalked him in recent seasons, particularly his back problem, have led to the situation where he now finds himself in real trouble if he progresses beyond the 18-yard line.
"If he was able to play every game defending the edge of the 18-yard box, as Chelsea did against Barcelona in the Nou Camp last season, Rio would be able to get the cigars out and play in cruise control.
"But he has to play a higher line for United because they are an attacking side and that means he is vulnerable whenever he comes up against somebody with pace."