Belgium coach Marc Wilmots will make alterations after his team struggled to a 2-1 victory over Algeria on Tuesday.
While Mertens could remain on the bench, Fellaini is likely to replace Nacer Chadli, while Thomas Vermaelen could start instead of Daniel van Buyten.
Captain Vincent Kompany is a doubt with a minor groin strain, while Wilmots must also decide whether to keep faith with Romelu Lukaku, who was ineffective during his 57 minutes against Algeria,
Russia failed to impress during their opening game with South Korea, but they still came away with a point that could potentially be vital at the conclusion of the group stages.
Striker Aleksandr Kerzhakov was introduced midway through the second half before netting the equaliser, and he could earn a starting role if boss Fabio Capello decides to make changes in his attack.
Possible starting lineups
Belgium: Courtois; Alderweireld, Kompany, Vermaelen, Vertonghen; Witsel, Dembele, Fellaini, De Bruyne, Hazard; Lukaku
Russia: Akinfeev; Ignashevich, Glushakov, Kokorin, Berezutskiy, Shatov, Zhirkov, Samedov, Fayzulin, Eshchenko, Kerzhakov
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Tag : World Cup