Jeremy Grantham, an investor credited with predicting the 2000 and 2008 downturns, told CNBC on Thursday that investors should get inured to lackluster returns in the stock market for the next two decades, after a century of handsome gains.

“In the last 100 years, we’re used to delivering perhaps 6%,” but the U.S. market will be delivering real returns of about 2% or 3% on average over next 20 years, the value investor and co-founder of Boston-based asset manager GMO told CNBC in a rare interview.