Former United defender Steve Bruce believes any successor to Ferguson will struggle to get close to emulating his record.
Hull manager Bruce said Everton boss David Moyes, the early favourite to replace Ferguson, had done well enough at Goodison Park to be able to take on the United job.
"He's cut out of the same cloth (as Ferguson) so it wouldn't surprise me," Bruce told Sky Sports News.
"Whether it's Jose [Mourinho] or David Moyes or whoever it is, it's going to be some act to follow.
"It doesn't seem quite right that Fergie won't be in charge of Manchester United any more. It's impossible to copy somebody like him.
Sir Alex Ferguson's brisk way with words has influenced the way we talk about football.
A timeline of the life of outgoing Manchester United manager Sir Alex Ferguson.
A special photo gallery of Sir Alex Ferguson's career at Manchester United as he steps down from his post as manager of the club.