"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.
"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."