In-form rookie Johnston to start for Rabbitohs in preliminary final against arch-rivals the Sydney Roosters.
South Sydney's all-time leading try-scorer Nathan Merritt has announced his retirement after 13 seasons and 236 games.
"I'm proud to have worn the red and green," Merritt said yesterday.
"It's been a remarkable ride ... something I never thought I would do."
Merritt broke Benny Wearing's long-standing Rabbitohs all-time try-scoring record with his 145th first grade try in round six against Penrith, but has played only two games since.