Sumerlin's comments, which came around midday, helped push stocks lower.

“To get someone who was part of the former economic council saying the Fed will need to step up big and do $6 trillion in (asset purchases), was a bit of a shock and created a bit of nervousness,” Marc Pado, market strategist at Cantor Fitzgerald, told CNBC.com.

Few economists expect the Fed to commit that much new money to helping the economy, and many think any further quantitative easing wouldn't have that much impact anyway.

Pimco co-CEO Mohamed El-Erian, for instance, told CNBCearlier Thursday that further monetary easing by the Fed and other central banks probably won't work.

"The risk is that this may be ineffective again," he said. "In fact, the big story of the last year and a half is every time we have had a policy action, outcomes have fallen short of expectations."

Raoul Pal, global macro analyst for the Global Macro Investor, who appeared with Sumerlin on CNBC, also questioned the wisdom of further easing.

"There is no evidence that it's ever worked in the past, so there is no real evidence that it will work now," Pal said. "So I think it's a high risk thing for them to try and do. I also don't believe you can get the money in the system. Even if it's $6 trillion, I don't think it's going to get in the system because there is no velocity of money. So all you end up doing is buying the Treasurys off the banks who will keep the money at the Fed."

As for extending the Bush tax cuts—which Congress has put off voting on until after the November elections—Sumerlin said "we will have a recession" if they are allowed to expire.

"Because we're growing too slowly," he said. "If you look at the quarters when the tax cuts went in, there was very substantial growth. People forget that the third quarter of 2003, we grew at 7 percent when the tax cuts—the full marginal rates—took effect. Running that in reverse causes the opposite to happen."