Stuart Lancaster is forced to play catch-up in ranking chase

Australia game now a must-win if England are to avoid potential group of death at World Cup

Stuart Lancaster is on record as saying that when he goes to bed at night, he does not lie awake worrying about England's position in the International Rugby Board's official rankings: indeed, he would not be alone in regarding the list as a guaranteed cure for insomnia. But the most mathematically challenging ratings production in the whole wide world of sport will come into its own over the next three weeks, which explains why the red-rose coach is giving serious consideration to changing a winning side for this weekend's meeting with Australia at Twickenham.

England may have beaten Fiji by a record score last weekend, but they dropped down a place in the rankings all the same. Only an IRB employee with a book of logarithms in one hand and a miniature version of the Large Hadron Collider in the other could explain the whys and wherefores of this apparent contradiction, but it has something to do with the unexpectedly heavy defeat suffered by the Wallabies in France. England have gone from fourth to fifth, while the Tricolores have done the precise opposite – something that may impact considerably on next month's pool draw for the home World Cup in 2015.

Lancaster's men now have three games to reverse that positioning, and as the final two matches of this autumn programme are against South Africa and the close-to-unbeatable New Zealand, there is no margin for error against the injury-weakened, internally ravaged Australians. If England finish this series outside the top four, they could find themselves in a "group of death" with the All Blacks or the Springboks, plus either Scotland or the ever-dangerous Samoans for good measure.

Hence the coach's talk yesterday of possible changes in a number of areas, including the back five of the pack, the midfield and the wing positions, for what is now a "must-win" match for all sorts of reasons. "We recognise the size of the challenge we face," Lancaster said. "There is bound to be a Wallaby reaction to the scale of their defeat in Paris, because they are a team of character and resilience. It is only a couple of weeks since they took the All Blacks all the way in Brisbane and emerged with a draw. They'll be a different animal this week, compared to last."

Assuming Lancaster grants Chris Ashton an immediate recall after the Saracens wing's one-match suspension for dodgy tackling and continues to back Charlie Sharples following the Gloucester player's excellent two-try contribution at the weekend, Ugo Monye of Harlequins will miss out. Other players pushing hard for starting places include the tough-minded Northampton flanker Tom Wood and the Gloucester No 8 Ben Morgan, who lost his place to Thomas Waldrom of Leicester during the summer tour of South Africa but is very much under consideration following Waldrom's failure to make the most of the runabout against a miserably disorganised Fiji.

The coach also has in his thoughts two London Irish players, the centre Jonathan Joseph and the loose-head prop Alex Corbisiero, along with the young Wasps lock Joe Launchbury, who made his international debut off the bench on Saturday. Joseph has recovered from an ankle problem and is expected to take a full part in today's training. Corbisiero, out of action for more than four months, played 63 minutes for his club on Sunday and is now considered fit.

"Graham Rowntree [the England forwards coach] went to see Alex play and was pleased with the way he went," Lancaster reported. "But that was his first start in months, so we have a decision to make on whether he needs more time. Besides, I thought Mako Vunipola [another debutant at the weekend] did very well for us against Fiji and he continues to be impressive in training. He's accurate in what he does and he's quick to understand what we want from him."

Manu Tuilagi failed to train yesterday because of a foot injury, although the coach said there was no reason to think he was struggling to make the Wallaby date. Another Leicester player, the lock Geoff Parling, also sat out after ending the Fiji game with a shoulder problem but is expected to be declared fit in good time for Australia.