"The most immediate thing for the FOMC members must be to think about what are the implications and quantifying the impact of this is just incredibly difficult. But it must be clear that if anything the impact is going to be negative," the chief international economist at Deutsche Bank said in an interview with CNBC's "Closing Bell."
The U.K. voted Thursday to leave the European Union, sending the U.S. stock market into a tailspin Friday. The Dow Jones industrial average closed down 610 points, with financials posting their worst day since 2011. The S&P 500 and Dow erased year-to-date gains and joined the Nasdaq composite in negative territory for 2016.
For now, central banks around the globe are in a wait and see mode, said Slok. The fact that they haven't done anything yet begs the question of how long the Brexit issue will hang around the U.S. economy, he said.
"Is that going to be a confidence effect that is going to be here for a long time or will it be that in a few weeks we just get used to it as a Brexit issue going on in the background and therefore confidence in the U.S. may move back up again?"
He also noted that this is a political crisis, not a financial one, so there is no need for easing at this stage.
"A political crisis has some whole different features around it. It will take a lot longer. It will be more unpredictable. A financial crisis is a credit crunch and that's just not what we're experiencing now."
— CNBC's Evelyn Cheng and Crystal Lau contributed to this report.