Marsh was speaking in Adelaide where he will take up a consultancy position with the South Australian Cricket Association beginning next week.
He has recently returned from England, where he was director of their National Cricket Academy.
Marsh said Australia relied too heavily on McGrath and Warne and urgent action needed to be taken to find prospective replacements for the pair.
"You can't take Warne and McGrath away from the Australian side, which is going to happen at some stage, and be the same side," he said.
Marsh was head coach of the Australian Cricket Academy (ACA) from 1991 to 2001.
He left the ACA to take up his position in England where oversaw the development of many of their Ashes heroes, including fast bowler Stephen Harmison.
Marsh said Australia was in desperate need of developing depth in their fast bowling ranks.
"You need a battery, I reckon, of 10 fast bowlers to choose from at any given point in time," he said.
"Obviously if anyone's going to break down, it'll be the fast bowlers so that's what you need, and that's what Australia must develop."