The pace bowler was rested for last week's three-match series in Zimbabwe, where Marsh steered the side to runaway wins in their first two games but then a nerve-wracking, last-gasp triumph in Sunday’s final match of the series.
Marsh will continue in the role as Cummins shrugs off jet lag and concentrates on preparing for the three-test series against South Africa that starts next month.
"Pat's main focus is the test group, and a lot of our bowlers and him in particular are preparing for that test series,” Marsh explained at Wednesday’s pre-match press conference in Durban.
"I'm hopeful that he'll feature in this series at some stage, but while he's not, I'm very happy to do it.”
Cummins has played two out of a possible 31 ODIs since leading the team to the 2023 World Cup title and has not appeared for his country in the 50-over format since a nine-wicket loss to Pakistan in Adelaide almost two years ago.
But the 33-year-old could return for one or both of the remaining two ODIs, at the Wanderers in Johannesburg on Sunday and in Potchefstroom next week.
The two ODI series in Zimbabwe and South Africa offer an ideal chance to get used to conditions for next year’s World Cup, which the two southern African countries co-host with Namibia.