Wallabies coach Ewen McKenzie has hailed cystic fibrosis sufferer Nathan Charles as a medical marvel after lining up a Test debut for the inspirational Western Force hooker.

Dumped Wallabies skipper James Horwill loomed as the feel good story for Saturday night's second Test against France in Melbourne after game-one injuries paved the way for him to celebrate his 50th match for Australia back in the starting line-up.

Horwill is in the run-on side for lock Sam Carter (ankle), NSW Waratahs hooker Tatafu Polota-Nau replaces Stephen Moore (knee) and Western Force workhorse Ben McCalman slots in at No 8 for Wycliff Palu (ankle).

However, McKenzie admitted Charles's tale was hard to beat - and explain.

Charles, believed to be the only cystic fibrosis sufferer in the world who has a professional career in a contact sport, is in line for a Test debut along with Melbourne Rebels pair Laurie Weeks and Luke Jones after their selection on the bench.

Charles, 25, requires up to 28 pills and vitamins every day to manage his condition - not that McKenzie could tell.

"I was talking to the doctor today - he [Charles] seems to defy science and logic," McKenzie said.

"I think if anyone had not said anything to us, we would not have known [about his condition].

"I always talk about journeys, how people get there and he's got a very unique story.

"I am glad it is out in the open and I think the best we can do to acknowledge him is to give him an opportunity on the field."

While impressed by Charles's triumph against the odds, McKenzie said the Force hooker had leapfrogged the more-established James Hanson in the Wallabies' queue completely on his own merits. - AAP