The stock market won't keep returning the kinds of yearly gains investors have gotten used to since the financial crisis bottom in 2009, Vanguard's chief investment officer, Greg Davis, said.

"If we look forward for the next 10 years, our expectations around U.S. equity markets is for about a 5 percent median annualized return," he told CNBC on Monday. "Five years ago, we'd have been somewhere in around 8 percent."

"Our expectations have clearly come down," Davis added. The historical average annualized return for the stock market, accounting for inflation, is about 7 percent.

The — which has soared about 15 percent since its Christmas Eve closing low, after three months of turmoil — is at the "high end of fair value," Davis said on "Squawk Box" from the Inside ETFs Conference in Hollywood, Florida.

Davis sees earnings growth slowing to somewhere in the single digits this year, after last year's much stronger rate. As for the economy, which is coming off what's expected to be about a 3 percent growth rate for 2018, Davis sees U.S. gross domestic product rising around 2 percent this year.

With stock market returns slowing as earnings and economic growth cool off, Davis said Americans are going to need to save more and save for longer.

Vanguard, the mutual fund giant founded in Valley Forge, Pennsylvania, in 1975, has about $5.3 trillion in global assets under management, as of Sept. 30, 2018.