You know economists are worried when they call for the Federal Reserve to institute emergency rate cuts. That's what Jeremy Siegel, a professor emeritus of finance at Wharton and chief economist at WisdomTree Investments, said Monday on CNBC's "Squawk Box."

"This may surprise people," Siegel said. "I'm calling for [a] 75-basis point emergency cut in the fed funds rate with another 75-basis point cut indicated for next month at the September meeting, and that's minimum." He added that the benchmark federal funds rate should be between 3.5% and 4%.

Siegel's reasoning was to follow previous Fed statements, such as the "long run fed funds rate when inflation reached 2% and unemployment has come up to 4.2% should be 2.8%," which would be "normal." He then said that inflation at 2.5% was more than 90% of the way to the first Fed target and the 4.3% unemployment rate was already past the second but with no rate cut.