There could be a lot more pain for the stock market if Leuthold Group's Doug Ramsey's historical valuation comparison becomes a reality.
"If we were to mark down the S&P 500 to the same P/E on trailing earnings that existed ... back in October 2007, the market would have to go down to 2,250," the Leuthold chief investment officer told CNBC on Friday.
"If you marked it down to the same price-to-sales ratio that existed in October of '07, the market would need to go down to 2,050," he added in a "Squawk Box" interview.
Based on Thursday's S&P 500 close of 2,650, Ramsey's price-to-earnings comparison would be a 15 percent decline. His price-to-sales comparison would be a 22 percent drop.
Ramsey said he could see declines of those magnitudes "over the next six to 18 months."
"I think it's very likely." he said. "And again, it's the point that I don't need to give you a draconian assumption to get that much down side."
The Dow Jones Industrial Average dropped more than 200 points on Friday, with weak economic data from China to blame. The S&P 500 opened back in a correction, measured by a decline of 10 percent or more from its most most recent high.
Shortly after its all-time intraday high of 2,940 on Sept. 21, the S&P 500 tanked 6.9 percent in October, its biggest one-month slide since September 2011. It bounced 1.8 percent in November, but went right back into the soup in December, losing about 4 percent so far this month.
Many Wall Street strategists remain cautious or even bearish like Ramsey, including Morgan Stanley's Michael Wilson, who sees a stagnant performance from stocks and the risk of an "earnings recession."
But there's a growing contingent of market watchers who are expressing optimism. UBS chief equity strategist Keith Parker told CNBC Thursday evening the S&P 500 could rise more than 20 percent by the end of 2019. Longtime strategist Jeff Saut of Raymond James predicted on CNBC Thursday morning the stock market has reached a bottom following a tumultuous 10 weeks.