As an FYI. IF it's the same over there as in the NRL, NSWRL and QRL.
If a player is injured in warm up the team doctor has to confirm he cannot play and then the opponent's team doctor, (if available) also has to confirm the same thing.
So in this case, it could be down to a muscular/neural problem making the muscle spasm therefore any further risk of complications would allow him to be replaced at late notice.