Steve Bruce believes Manchester United face an almost impossible task as they look to replace their ageing defence.
The former United captain, who takes his Hull team to Old Trafford on Tuesday, believes that Louis van Gaal, the probable next manager, has a huge rebuilding job.
And Bruce, a cornerstone of United's defence between 1986 and 1996, feels the hardest part is finding defenders of the calibre of Nemanja Vidic, Rio Ferdinand and Patrice Evra.
The trio, who have been teammates since 2006 and were part of the 2008 Champions League-winning side, are all out of contract in the summer.
Vidic has already announced he will join Inter Milan, Evra said he will decide his future in two weeks' time and Ferdinand is the subject of reported interest from clubs in Turkey and the United States.
And Hull manager Bruce said in a news conference: "It is a global club and a really big job and whoever comes in is going to have to live up to the standards set by the former boss [Sir Alex Ferguson].
"It is a hard job, though, because time catches up with everybody and that is there to see with the players in United's back four.
"Patrice Evra, Rio Ferdinand and Nemanja Vidic have been in United's back four for almost 10 years and how do you replace them? No matter how many millions you have, it is not going to be easy to find players to come in and replace them. #
"In Vidic and Ferdinand alone, you have two centre-halves with years of experience that you just cannot find easily."
Netherlands manager Van Gaal is the favourite to replace David Moyes and Bruce added: "Everybody is saying it is going to be Van Gaal and the guy has a fantastic record in management."