"Whoever is appointed, the dynamic [is] if you're going to execute fiscal policy, you need to keep rates low for a long time," the BlackRock chief investment officer of global fixed income said on "Squawk Box."
Before joining the administration, Cohn was the No. 2 executive at Goldman Sachs. A Cohn appointment would break with a tradition of having an economist as Fed chief.
Rieder, who oversees $1.3 trillion in fixed income assets at the world's biggest money manager, said it's unclear who might be selected to lead the Fed. But the appointment choice probably won't be dramatic, he said.
"It tends to be a normal transition," he said. "Quite frankly, part of why the Fed is executing this reduction of the balance sheet — you're getting ready for the new Fed chair."
Rieder said the equity market will continue to climb because "the discount rate in the world will be held down by the central banks even while the Fed normalizes, [and] even while the ECB normalizes."
Last month, the U.S. central bank said it will begin the process to reduce its $4.5 trillion "balance sheet," a portfolio of mostly government debt accumulated in the years after the 2008 financial crisis.
The Fed, in its June minutes, reiterated its federal funds rate forecast of 1.4 percent by the end of 2017. The Fed has raised rates only four times in the last 10 years — in June and March of this year and in December 2016 and December 2015.