Investors who buy now "won't be sorry" a year from now after the market sell-off, according to Wharton professor Jeremy Siegel. "We've had bigger shocks in the past...There may be another 5%, who knows, there may be another 10%, but that means for me, moving forward, that just raises the return on the market looking forward," Siegel said on CNBC's "Squawk Box." "Hold in there. If you got cash, begin to employ it. You won't be sorry a year from now," he continued. The finance professor, author of 1994's " Stocks for the Long Run," made his remarks as stock futures tumbled Monday, setting the S & P 500 on a path to return to bear market territory after last week's losses. On Friday, the broad market index closed down 19% from its record high set in January. Still, Siegel believes that the S & P 500 is poised to beat inflation, which it has historically done over the long term, even as prices have so far failed to peak. The professor said the broad market index is currently lagging the consumer price index. "Through history, the S & P has beat the CPI by 4 to 5% a year. So really, it is way below its lag. So don't forget in the long run, the evidence is we're going to overcome that inflation with this stock index. It's lagging its historical performance," Siegel said. "There may be downside... but let me guarantee you when this is over, the S & P will jump ahead of what the consumer price index is. That's always the way it's been historically," he said.