He steps in for Jared Payne, who is out injured because of groin strain, while Michael Allen will make his first European start on the left wing.
Mark Anscombe made six changes for the game at Stade Pierre-Antoine including moving Paddy Jackson to the bench as Ruan Pienaar starts at 10.
The South African will be partnered by Paul Marshall, wearing number nine.
Iain Henderson has been moved out to allow Neil McComb to make his European debut.
Roger Wilson will step in at number eight for Nick Williams, who is still recovering from a knee injury.
A win would secure Ulster's first ever home quarter final in Europe and their only victory to date on French soil.
But with Castres winning three of their last four home games in the Heineken Cup, Anscombe's side will need to be playing at full par to come out on top.
(15-9): C Gilroy, A Trimble, D Cave, P Wallace, M Allen, R Pienaar, P Marshall.
(1-8): T Court, R Best, J Afoa, L Stevenson, N McComb, R Diack, C Henry (capt), R Wilson.
Replacements (16-23): R Herring, C Black, D Fitzpatrick, I Henderson, M McComish, M Heaney, P Jackson, C Cochrane.