The 30-year-old ended a tumultuous week, in which he was arrested in the early hours of Monday morning following a scuffle at a Manchester nightclub before being released without charge, by limping off the field shortly after half-time in Australia's 62-0 thrashing of the United States at the Glyndwr University Racecourse Ground.
Sheens confirmed Slater had suffered a recurrence of a posterior cruciate ligament injury, which ruled him out for several weeks last year.
Yet the Australia coach was quick to quieten down suggestions Slater is now out for the tournament, although he did concede the Melbourne and Queensland representative will miss the semi-final encounter with either Fiji or Samoa next week and will undergo an MRI scan on Saturday night.
"It's an injury to his old injury - PCL," said Sheens. "It's just what damage may have been done, cartilage and other things.
"We're just hoping it's a bad knock but we're sending him for an MRI tonight.
"We've still not given up the thought that he might possibly be available, probably not this week after that sort of knock, the week after if we get that far."
Centre Greg Inglis, who scored two tries as Australia cantered into the last four at the expense of the United States, is the favourite to fill in for Slater at full-back.
"We've got a few full-backs actually who can play the game. It's a still a big loss for us," added Sheens.
"I'm not going to panic just yet until we get (the results of the MRI).
"The issue is the knock and how much damage it's done."
Brett Morris and Jarryd Hayne touched down with four tries apiece, while there were also scores for Cameron Smith and Cooper Cronk, as tournament favourites Australia sent a warning to their rivals with a 12-try showing to ruthlessly end the challenge of surprise packages the United States.
Yet it was Australia's defensive showing, with the Kangaroos keeping a clean sheet for the second successive game following a 50-0 victory over Ireland in their final group game, that pleased Sheens.
"I think we showed plenty of determination in defence. I think we have the best defensive record in the competition at the moment," he said.
"We had our pants pulled down early by England (in their opening game) and we've determined that won't happen again, so we've really worked hard on that aspect of our game.
"It comes behind a kick-chase game. I was happy with the way we continued to put the pressure on the USA."
Australia will be confident of booking their place in the final on November 30 at Old Trafford after earning their fourth victory from as many matches in the competition.
They will also be buoyed by a 34-2 victory over potential last four opponents Fiji at Langtree Park earlier this month.
Yet Sheens will not rest on his laurels and will take the threat of either Fiji or Samoa seriously.
"Obviously, we've played Fiji. They're both big strong sides so we'll have a very physical game next week in front of us," he added.
"And particularly when it's sudden death, one game away from the final, there will be plenty of intensity."
Tomahawks head coach Terry Matterson was proud of his side despite their efforts in Wrexham.
The United States were given a 50-point head start by most bookmakers in the handicap coupon and it was clear to see why on occasions.
Nevertheless, they won plenty of fans in this tournament and reached the last eight courtesy of spirited performances against the Cook Islands and Wales in Group D.
When Matterson was asked how he felt, the former Castleford coach responded: "I suppose a bit of relief that that game's over, they're a wonderful side that we played against.
"I'm no less proud of these boys for what they've done. We're focusing on what we've done in the campaign and reaching the quarter-finals.
"What we've done over the last four weeks has been very special and I'll always hold those players to my heart.
"I'll always remember and we'll always have a very strong bond."
- Sports & Recreation
- United States