Moses pulled up sore while running the ball during a training session at NSW's Blue Mountains camp on Thursday, injuring the same calf that troubled him earlier this year.
The Blues have sent Moses for scans on the injury that delivers a blow to their hopes of sealing a series victory by winning Origin II in Perth on Wednesday.
"It's terrible, isn't it? Mitch played really well in game one," said Blues coach Laurie Daley.
"We're unsure of how bad it is but he won't be any good for Wednesday with the short turnaround."
Daley called Luai on Thursday to ask whether he would be available after being beaten for selection in the series opener by Moses.
Luai, who had just finished on the training paddock at Wests Tigers, answered Daley's distress call to reunite with former NSW halfback Nathan Cleary in the halves.
The pair won four premierships together at Penrith and have featured together at Origin level six times already, with Luai also featuring as the Blues' five-eighth in all three games last year.
"(Luai) played there last year, has got a good combination with Nathan and he's that left-side player," Daley said.
"He'll do a fine job."
The NRL has already granted the Blues an exemption to bring Luai into the team from outside the 20-man squad that Daley had named on Tuesday.
It means Canterbury playmaker Matt Burton, considered the other contender to replace Moses, will remain 18th man as initially planned.
Luai will arrive at the Blues' team hotel in Leura on Thursday afternoon.
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