Fellow tighthead prop Jamie Corsi has also been ruled out, after a back injury sustained in the gym last week failed to clear up in time, leading Wales to call for Horsman to fly down under.
Ceri Jones, who can play on both sides of the scrum, takes his namesake's place in the front row and Horsman will be asked to provide cover from the bench when he arrives in Brisbane at 10.30am tomorrow (Friday) morning.
"Adam has a small tear to his right calf muscle which will prevent him from training now for three to four weeks," said Wales Team Doctor John Williams.
"Unfortunately, Jamie sustained a lower back injury in the gym last week which has not cleared up in time to make him available for selection and so we have arranged for Chris to fly out.
"The protocol for him playing rugby after a long haul flight like that will need to be managed and we will be giving him all the correct advice when he arrives.
"Jet lag shouldn't be a major issue, but we will try to get him to sleep in as much as possible on matchday as tiredness will probably be his main problem if he has one. The actual effects of the flight on his body clock probably won't have had time to affect him because the game comes so soon after he lands and takes place at 11am UK-time."
Adam Jones has backed his former understudy to rise to the challenge presented by Australia on Saturday night. "Ceri will go alright, there's no worries about that," he said.
"He went well when he came on last week and he's got what it takes to handle the challenge on Saturday. I'm obviously gutted to be missing out, but I'll be wishing Ceri all the best before the game and I'll be right there for him if he needs anything from me."
Chris HORSMAN (WORCESTER WARRIORS)
DoB: 02.02.78. Ht: 6'3"/1.91m. Wt: 17st 6lbs/111kg
International record: 9 caps
Wales: Gareth Thomas (Capt); Chris Czekaj, Jamie Robinson, Sonny Parker, Aled Brew; James Hook, Mike Phillips; Iestyn Thomas, Mefin Davies, Ceri Jones, Michael Owen, Rob Sidoli, Colin Charvis, Gavin Thomas, Jonathan Thomas
Replacements: Chris Horsman, Richard Hibbard, Scott Morgan, Robin Sowden-Taylor, Andy Williams, Ceri Sweeney, Gavin Henson
Australia: J Huxley, D Ioane, S Mortlock (Capt), A Ashley-Cooper, D Mitchell; S Larkham, M Giteau; W Palu, G Smith, R Elsom, D Vickerman, N Sharpe, G Shepherdson, S Moore, B Robinson
Replacements: A Freier, M Dunning, M Chisholm, S Hoiles, P Waugh, G Gregan, M Gerrard