It's an open secret. Whether or not he will be his immediate successor is another question.
Mourinho won't leave until he has won the league in Spain or is fired and if Mourinho manages to find somone other than Higuain and Ronaldo to score goals and becomes successful then we will either see Fergie in place for a few more years or United being led by intermediary until Mourinho becomes avaliable.
The more interesting question is who will be in the position if Mourinho isn't instantly avaliable and Fergie doesn't want to do "one more season". It couldn't be someone too big like Capello, Van Gaal or Hiddink for obvious reasons and Moyes is probably their long-term solution after Mourinho. The likely candidates are probably Phelan, Bruce, Queiroz or Hughes; probably with Gary Neville as assistant.
Whoever the intermediary will be it would be the worst job in football. No money would be avaliable, all transfers would go through mourinho anyway, expectations will be high, morale will be low (after Fergie's exit) and all the young stars who have been nurtured by Fergie won't stick around because the United PR machine will lie and say that whoever is in charge is "here to stay".
Nope, it will be a mess... Unless some Emir from the middle east buys the other half of the club, that is...