Queensland selectors held a meeting on Monday and had been set to name both Slater and Harrison before the Titans announced he was out.
Brisbane lock Corey Parker will now make his Origin starting debut in Harrison's spot while his Broncos teammate Ben Te'o will be called into an extended Maroons squad.
Slater would seem long odds to play given medical opinion he could do more damage to his knee.
But given his match-winning brilliance, Queensland are willing to give him every opportunity to get on the field.
Slater, who is understood to have had a cortisone injection in his knee, is likely to continue his extensive physiotherapy treatment in Melbourne until Wednesday or Thursday when he or Queensland's medical staff will have to make the call.
Former Queensland captain Darren Lockyer, faced with a similar tough decision in 2008, said Slater would make the right call.
"Everyone's going to have an opinion, including the doctors. But at the end of the day Billy will know whether he can play and I'm sure he'll make the best decision for the team," he said.
"It'll be a tough decision for Billy, but if he knows he's not capable of playing a physical 80 minutes at that level, then I don't think he would take the field," he said.
"But if he does, then you can be confident he'll do well."
The player under most pressure is giant South Sydney forward Dave Taylor who'll lose his starting spot to Sam Thaiday.
And with Te'o in the squad his place on the bench is not secure.
Canberra prop Dave Shillington, the players' player off the bench in Origin II in Sydney, is in the mix to start in the decider to help take some sting out of an aggressive NSW pack which has had the better of Queensland early in both games.
Starting front-rower Matt Scott will attend his mother's funeral on Wednesday and his emotional state will be considered when the final decision on the makeup of the starting side is made.
The Queensland team will be announced at Coolum this afternoon.
-AAP
