If not for concerns about Europe, the U.S. stock market would be 20 percent to 25 percent higher, Jeremy Siegel, economics professor at the University of Pennsylvania's Wharton School told CNBC Thursday.

"We're all waiting. Will there be a Lehman-type event? But I don't think so, because the European Central Bank knows, reluctantly, it will do what it has to do before we get to that stage," he said.