Rookie McDermott, the step-son of former test bowler Craig McDermott, could make his ODI debut should Marsh fail to recover in time for the second match in Adelaide on Friday.
"We will continue to monitor Shaun's progress and give him every opportunity to prove his fitness ahead of the match," selector Trevor Hohns said in a team release on Tuesday.
The 23-year-old made his international debut in a Twenty20 match against United Arab Emirates last month and played all three of Australia's T20 defeats against Pakistan in the Gulf nation.
Aaron Finch's Australia lost the opening one-dayer against South Africa by six wickets at Perth Stadium. The third and final match is in Hobart on Nov. 11.
(Reporting by Ian Ransom; Editing by Pritha Sarkar)
