The Kangaroos and Kiwis face the prospect of reaching the final with 23 men following tournament-ending injuries to Jake Trbojevic (pec) and Gerard Beale (ankle) respectively.
It is unlikely they will make requests to draft in a replacement player given there are already members in their squad capable of filling in the vacant position.
The Lions also copped a blow with star forward Sam Burgess facing at least two weeks out with a knee ligament injury picked up in their 18-4 loss to Australia on Friday.
The tier one nations aren't the only teams forced into changes for their next pool match, with Fiji losing towering prop Kane Evans to a broken wrist in their rout of USA on Saturday.
Trbojevic, Beale and Evans look like missing all the pre-season for clubs Manly, Warriors and Parramatta respectively.
On Sunday, Italy's hopes of progressing through to the knockout stages were dashed when veteran Terry Campese was ruled out before kick-off with a calf problem.
Halves partner Jack Johns was then sent to hospital with a suspected broken arm during the match, stripping coach Cameron Ciraldo of his first choice halves.
The Azzuri now have to put an unlikely cricket score on USA before upsetting Fiji to be any chance of making the quarter-finals.
Ciraldo has to turnaround a side that were pummelled 36-12 by Ireland on Sunday.
"Jack's got an arm problem, he's at hospital now having it checked out. Hopefully he will be right," Ciraldo said after the Irish loss.
"We tried to prepare without Campo as well and we were ready for that situation.
"There's no excuses, we weren't good enough today."
The injuries soured a string of notable performances by minnow nations across the opening round of group matches, with Papua New Guinea, Fiji and Tonga all cracking 50 points.
The Kumuls spanked Wales 50-6, the Bati scorched USA 58-12, while Jason Taumalolo and Andrew Fifita helped Tonga smash Scotland 50-4.
Tournament favourites Australia were rusty in an unconvincing 14-point win over England, while New Zealand showed some life in a 38-8 trouncing of a strong Samoa outfit.
In arguably the closest Test of the weekend, Lebanon edged France 29-18 in Canberra to register their maiden World Cup win.
