MACAU—Casinos in the world’s richest gambling market were already down on their luck when city officials ordered them on Tuesday to close their doors at midnight for 15 days to help curb the spread of the coronavirus that has killed hundreds of people in China.

There were more staff than punters inside the red- and gold-carpeted floor of Sands casino here on Tuesday afternoon as Macau Chief Executive Ho Iat-seng discussed the unprecedented two-week shutdown with casino operators including Wynn Resorts Ltd. and Sheldon Adelson’s...