Investors should expect more headwinds as the Nasdaq descends into a bear market, Wharton School finance professor Jeremy Siegel predicted on CNBC on Monday. Siegel told " Squawk Box " that if the Nasdaq were to go down another 10% or 15% that would "not be unusual" and investors should brace themselves for an even more tempestuous market. "When a bear market comes, it doesn't spare the good stocks or the bad stocks. I mean, they all go down," Siegel said. A bear market is defined by declines of 20% or more from recent highs. "I think there's more pain to come," he added before the market opened to the downside. As Siegel was being interviewed, U.S. stock futures on Monday were indicating more big losses following the Nasdaq's and the S & P 500 's worst weeks since March 2020, when the Covid pandemic's onset rocked the market. The Nasdaq was firmly in correction territory last week, down 14% from its November record close. The S & P 500 and the Dow Jones Industrial Average were down 8% and 7%, respectively, from their record closes earlier this month. The S & P 500 will likely go into correction territory — down 10% or more from recent highs — due to recent hits against growth stocks, Siegel said, adding value stocks will outperform growth stocks with a deeper rotation than the one seen from October 2020 to March 2021. The Wharton professor forecasts the Federal Reserve will raise interest rates eight times this year, double the hikes the market expects in 2022. The central bank's two-day policy January meeting is set to start Tuesday. No change in rates is expected this month. The first Fed rate increase to fight surging inflation is expected in March. For months, Siegel — who's been correctly bullish in recent years — has been worried about rising inflation and a more aggressive Fed. Investors should hold off on investing their cash until the market bottoms more, Siegel said. But when they do, he said, they should buy stocks in profitable companies, echoing "Mad Money" host Jim Cramer 's advice to invest in companies with tangible products and results. "When the reaction and pessimism is over, those that are good stocks bounce back. And those that are not, do not bounce back," Siegel said. However, he warned, "There's no way to completely safely weather this storm."