"It's too difficult to tell at this early stage exactly how to forecast it," he said about the coronavirus. "We are still operating our stores [in China]. Almost all of them are open ... but operating on reduced hours," with a focus on selling food and consumables, he said.
Walmart has 430 locations in China. It also has a minority stake in JD.com, one of China's leading e-commerce players.
"Shipping and product coming out of China ... into the U.S. ... is an issue," McMillon said. "It's too difficult to call right now exactly what will happen in the [first] quarter. But we've said, because of what's happening on the ground in China ... we do expect to have some impact. But we are not currently putting that into our guidance."
Looking to full-year fiscal 2021, Walmart said Tuesday that it is calling for earnings to fall within a range of $5.00 to $5.15 a share. Analysts had been calling for annual earnings of $5.22 per share.
Its fourth-quarter earnings came up short of analysts' expectations, as the company said there was weakness in toys, video games and apparel during the holidays.
Other retailers ranging from Ralph Lauren to Under Armour have already said they expect to take a sales hit because of the coronavirus. Many high-end retailers are expected to struggle as Chinese tourism and spending on luxury goods drops off. Factories have been shut down, and it's unclear when many will be back up and running again.
Apple said Monday that it does not expect to meet its own guidance for the March quarter because of the impact from the coronavirus outbreak.
Walmart shares were last up less than 1%. The stock has climbed almost 20% over the past 12 months. Walmart has a market cap of about $334.5 billion, according to FactSet