I'm not up to date on FM terminology so perhaps I've not been clear in calling Fellaini a DMC. I never meant to imply that he's going to be a replacement/stand-in/substitute for Michael Carrick (though of course he offers that as well). Primarily I see him as a replacement for Tom Cleverley, or perhaps more accurately a replacement for Darren Fletcher.
Consider the following two United lineups:
van der Sar
Neville - - Rio - - Vidic - - Evra
Carrick - - Fletcher
Ronaldo - - - Tevez - - - Nani
Rooney
de Gea
Rafael - - Rio - - Vidic - - Evra
Carrick - - Fellaini
Valencia - - - Rooney - - - Welbz
van Persie
I believe both the above setups as being comparable in the various roles and tactical arrangements. Perhaps Fletcher would have been a little more mobile than Fellaini, but the latter makes up for that with a superior physical presence. In either case the role of Fletcher/Fellaini would be to disrupt opposition play in the defensive phase and win the ball before passing it to a more attack-minded teammate, whilst in the attacking phase offering the simple option as an out for a teammate running into trouble. In either case the players would be expected to get back to help in front of the defence as well as roam forward to the edge of the box when on the attack.
I believe this is predominantly the role Fellaini will play for United and I don't see any reason why he couldn't fulfill it at least as well as Darren Fletcher did during the Ronaldo era.