Uncertainty also surrounds the availability of Justin Hodges (ribs), Sam Thaiday (shoulder) and Matt Scott (family reasons), prompting Maroons selectors to name a 20-man squad for the series decider in alphabetical order.
In what looms as Queensland's most-disrupted preparation, Queensland coach Mal Meninga said he would give Slater until Saturday "at the latest" to make a call on his troublesome knee.
Medical staff initially expected Slater to be sidelined for up to six weeks when he went down in game two on June 13.
However, Meninga held more than a flicker of hope that Slater would run out on July 4 onto a packed Suncorp Stadium as Queensland look to seal a seventh straight series victory.
Slater will remain in Melbourne working with Storm medical staff and will only go into Maroons camp on Queensland's Sunshine Coast if he rates himself a chance.
"Billy ran yesterday morning and he was very upbeat about that session so we are going to give him until the end of the week to make that final decision - Saturday at the latest," Meninga said.
"At this stage, we are very hopeful, but uncertain.
"Billy is desperate to play so that's why we will give him that extra time - he deserves that respect."
Asked who would make the final call on Slater, Meninga said: "Billy is going to be the best judge - I will leave it in his very capable hands.
"If Billy says he is right to go, he is right to go."
But Meninga added: "To play in this side, we need to be fully fit, or near fully fit.
"To carry an injury into this arena, if you are carrying an injury, you let the team down."
Greg Inglis looked set to move to fullback, with the Storm's Dane Nielsen to come into the centres, but Meninga did not rule out slotting in the in-form North Queensland No.1 Matt Bowen.
Meninga also did not rule out keeping Bowen on the bench with Inglis at No.1 if Slater succumbed to injury.
"Could be, we might not go with a full forward bench. That's another decision we have to make during the week," he said.
On Nielsen, Meninga said: "We rate Dane highly - he hasn't let the side down before."
There is also uncertainty around Scott, who will attend his mother's funeral on Wednesday.
"We just have to see where Matty is at the end of the week - there are some concerns there whether he is right to go," Meninga said.