Mendes will be named on the bench for the first time since sustaining a knee injury back in October, while Lafferty should also be involved after recovering from flu.
However, the Northern Ireland international will be the only recognised first-team striker available to manager Walter Smith.
Kris Boyd is recovering from a hernia operation and Kenny Miller is out with a hamstring injury, while Steven Naismith (hamstring) and Nacho Novo (ankle) will not return until next week.
DaMarcus Beasley (thigh) is also out this weekend and fellow American Maurice Edu (ankle) is a doubt, while Madjid Bougherra remains on international duty with Algeria at the African Nations Cup.
Gers (from): McGregor, Whittaker, Papac, Weir, Broadfoot, Fleck, Wilson, Davis, Thomson, Edu, Lafferty, Smith, Loy, Little, Ness, Mendes, Alexander, McCulloch.