Rio Ferdinand has admitted he does not know how Manchester United will be able to replace manager Sir Alex Ferguson when he eventually retires.
Ferguson, who celebrates a half-century in the game on Saturday, the 50th anniversary of his playing debut for Queen's Park, has said he plans to retire within the next two years.
"They're big shoes to fill, aren't they?" Ferdinand said in the Daily Mirror. "Who's going to fill them? I wouldn't like to say, but the club will have to work really hard to find someone capable of taking over."
He added: "It's not like bringing in another player, it's a different dimension altogether.
"Our manager's record is untouchable, so to replace someone like him after so long is going to be really difficult. But if any club can it's United."
Ferdinand added that Ferguson's fiery temper has eased a little of late, but warned that the Scot can still be set off at a moment's notice.
"The boss has mellowed since I've been at United but don't believe the hype too much," he said. "When it's time for home truths to be said, he'll tell you, in no uncertain terms, right between the eyes."
Even though no one is sure when Ferguson will retire, Ferdinand said the players were determined to keep winning trophies to end his reign on a high note, and backed the team to get back into the title race even though they are eight points behind Chelsea and Liverpool already.
"I have been in both positions, leading from the front and playing catch-up and there's pressure in both situations," he said.
"United have lost titles from being loads of points ahead and won it from being a long way behind. So I don't think it matters at this stage of the season where we are. But we know we can't afford any more slip-ups."
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