Fellaini has long been a familiar face in the Red Devils' set-up, but Januzaj is in line to make his first senior international appearance, having only committed himself to Belgium in April.
Januzaj's heritage meant he was also eligible to turn out for Albania, Kosovo, Turkey, Croatia and Serbia. But following months of speculation, the teenager chose to play for Belgium, the country of his birth.
While coach Marc Wilmots has named 24 players in his provisional squad, FIFA will require that to be whittled down to 23 men by 2 June, with one of Wilmot's four goalkeepers likely to miss the cut.
Belgium's provisional 24-man World Cup squad: Courtois, Mignolet, Proto, Casteels, Kompany, Vermaelen, Vertonghen, Alderweireld, van Buyten, Lombaerts, Vanden Borre, Ciman, Fellaini, Witsel, Defour, Dembele, de Bruyne, Chadli, Lukaku, Hazard, Mertens, Januzaj, Mirallas, Origi.