A nightmare week for the Kangaroos fullback continued when he was escorted off the field after hurting his left knee early in the second half of a 12-try romp in Wrexham, Wales on Saturday.
Scans have proved inconclusive due to swelling around the knee and Slater will continue treatment in the coming days, however it looks increasingly unlikely he will be able to return in time for the final on November 30.
Brett Morris and Jarryd Hayne scored four tries each, matching the record for an Australian in a Test match, as the Tomahawks' World Cup fairytale came to a brutal end and the Kangaroos set up a semi-final showdown with either Fiji or Samoa.
Slater, who was arrested and released without charge after being involved in a nightclub fight in Manchester on Monday morning, suffered the injury after fielding a bomb and copping a heavy shot from US forward Roman Hifo.
It was the same knee in which the Queensland star tore his posterior cruciate ligament in last year's State of Origin series, an injury which forced him out for almost six weeks.
The 30-year-old was clinging to hope after the match his World Cup was not over.
Being ruled out would be a cruel blow for Slater and deny him a chance at redemption following Australia's loss to New Zealand in the 2008 final.
"I'm pretty sure if the scans come back bad it'll be the tournament over for me," the 30-year-old said.
"Hopefully I'm a chance of playing again."
Asked whether it was a tough week, Slater said: "Sure is."
Australia at least have no shortage of fullbacks if their first-choice No.1 is ruled out with Greg Inglis and Hayne at their devastating best playing in the centres on Saturday.
South Sydney star Inglis scored a double against the Tomahawks, including one effort in which he out-muscled four defenders, and will play in his pet position in Slater's absence next week.
"He doesn't mind playing there either does he," Kangaroos coach Tim Sheens said.
"He comes to life at fullback."
Hayne justified his surprise selection at centre ahead of specialists Brent Tate, Josh Morris and Michael Jennings but Slater's injury will likely pave the way for one of the omitted trio to replace Inglis at left centre.
Hayne and Morris wreaked havoc on the right-hand side on Saturday, becoming the seventh and eighth Australians to score four tries in a Test match.
Morris scored his four tries inside 18 minutes in the first half as Australia led 38-0 at the break in front of 5762 fans.
Skipper Cameron Smith and halfback Cooper Cronk also scored four-pointers for Australia while Johnathan Thurston had an off first half with the boot before recovering to convert seven of Australia's 12 tries.
After playing two of their opening three games at night in cold, slippery conditions, the Kangaroos relished the dry, day-time affair in their most polished performance of the tournament so far.
"It was great to get (the outside backs) some quality football," Smith said.
The Tomahawks, who defied 500-1 tournament odds to reach the last eight, tried hard but the Kangaroos did not concede a try for the third straight match.