The golden glow around the Wallabies after their breakthrough Rugby World Cup win over Georgia in Paris has turned dark, along with the mood of coach Eddie Jones, with giant second-rower Skelton doubtful to take on Fiji on Sunday (Monday AEST) in Saint-Etienne due to calf complaint.
Australia will already be without spearhead prop Taniela Tupou due to a hamstring injury and first-choice halfback and vice-captain Tate McDermott after a head knock.
The Wallabies will give Skelton until kick-off to recover, with the Australians able to all but secure a quarter-final berth with victory over their pool rivals.
Another lock Matt Philip could be his replacement or Jones might turn to backrower Langi Gleeson, who came off the bench against Georgia.
In his fourth World Cup, James Slipper has replaced Tupou at tighthead while another veteran, Nic White, takes over McDermott's No.9 jumper.
But 23-year-old Frost was upbeat, the Brumbies ace excited to return to action after missing the last two Tests due to an ear injury.
Frost has had his ear reattached three times after he tore it from his head during the first Bledisloe Cup in July, as well as a brief stint in hospital when it became infected.
He had to fashion protective headwear from a water-polo cap for training, but is confident the injury won't affect his game.
"It's been really annoying because it's such a small thing which keeps you out but it's good to get a run this weekend," Frost said.
"I'm just trying to protect it as I've re-opened it a few times - every time you do contact and hit the side of your head it opens up.
"It's ripped through the webbing so pretty much after the tournament we'll get something done to fix it back up because at the moment, it doesn't really stay on the side of my head."
The Wallabies have targeted this match, looking to secure a spot in the play-offs before their next clash with Wales, who held on to beat Fiji in a thriller 32-26 in the opening pool round.
They have only lost to the Pacific island nation twice in history, the last time back in 1954 in Sydney.
Frost said the players were aware of the importance of the match.
"We've spoken around how this is probably our most important game," the Sydneysider said.
"First week of World Cup everyone's excited and it's a settling period but week two is when it really matters.
"We're all in for this game and we're really looking forward to it.
"The Fijians have been great; they've only lost two matches this year, they've had great wins and a close loss last weekend so they'll be fired up."
With a joint weight of around 270kg, the loss of Skelton and Tupou would dent the Wallabies' plans to dominate Fiji at set piece as they did against the Georgians.
Coach Jones said it was still an area they would go after.
"When you play against Fiji you'd always like to have a set-piece advantage and we've started the tournament off well in that area, so that's an area we'd like to keep growing in," Jones said.
