A surplus of second-rowers has veteran James Horwill, the Wallabies' 2011 World Cup skipper, seemingly in a three-way battle for the last remaining lock spot with injury-plagued stars Sam Carter and Kane Douglas.
Will Genia is also believed to be a touch-and-go selection, but is likely to make the plane to Britain because Cheika is struggling to identify his first-choice halfback.
Genia was given first crack at the No 9 jumper but limped off with a knee injury during Australia's Rugby Championship opener against South Africa, before Nick Phipps failed to fire against the All Blacks.
Nic White's late heroics in the Wallabies' victory over New Zealand in Sydney looks to have earned him a ticket after the Brumbies half fell down the pecking order last year.
Despite Matt Giteau covering for White last Saturday night in the second Bledisloe Cup clash in Auckland, Cheika is expected to name three halfbacks in his World Cup squad.
That being the case means Cheika must lose at least two of seven wingers included in a total of 40 players named in squads during Australia's four domestic tests.
With Adam Ashley-Cooper - the Wallabies vice-captain and premier backline utility - guaranteed selection and Rob Horne almost certain to go, Cheika and assistant coach Stephen Larkham must decide on two of Joe Tomane, Henry Speight and Drew Mitchell as other specialist wingers.
Despite bagging a hat-trick of tries for a World XV in a 45-20 win over Japan in Tokyo on Saturday, Taqele Naiyaravoro and fellow cult hero Nick Cummins are likely to be overlooked as Cheika leans towards a 14-17 split of backs and forwards.
He was among Australia's top-three performers at the last World Cup, but it appears James O'Connor will be overlooked without even getting a look-in for a wing spot this time.
Neither Bernard Foley or Quade Cooper have set the world on fire this winter, but selectors have little alternative than to choose both as Australia's two specialist five-eighths.
Veteran Matt Giteau and Matt Toomua will get the nod as fellow midfield players ahead of Christian Lealiifano, with Kurtley Beale also to be rewarded for his versatility and creativity.
The back-rowers mostly pick themselves, with champion breakaways Michael Hooper and David Pocock and rugged blindside flanker Scott Fardy all certainties.
Ben McCalman will also be picked, while Wycliff Palu is likely to edge out Scott Higginbotham as the squad's only specialist No.8.
Settling on four second-rowers is causing selectors the biggest headache.
Rob Simmons, Will Skelton and Dean Mumm, who has the advantage of being able to cover Fardy's No.6 role, are all set for inclusion.
But unless the Wallabies only take two hookers and four props, big gambles for rugby's showpiece in Britain where scrum dominance will be crucial, that leaves only one more second-row slot available.
That presumably leaves the decorated Horwill fighting for a boarding pass with Douglas and Carter, the two outstanding locks in Australia last year but who have battled long-term injuries in 2015.
Douglas looms as the big selection wildcard given he was lured back from Ireland mid-contract this month to return to Australian rugby ranks.
Possible Wallabies Rugby World Cup squad:
Backs - Israel Folau, Adam Ashley-Cooper, Rob Horne, Drew Mitchell, Henry Speight, Tevita Kuridrani, Matt Giteau, Matt Toomua, Kurtley Beale, Bernard Foley, Quade Cooper, Nick Phipps, Nic White, Will Genia.
Forwards - Wycliff Palu, Ben McCalman, Michael Hooper, David Pocock, Scott Fardy, Dean Mumm, Rob Simmons, Will Skelton, Kane Douglas, James Slipper, Sekope Kepu, Scott Sio, Greg Holmes, Tetera Faulkner, Stephen Moore (capt), Tatafu Polota-Nau, James Hanson.
