"Our player Kylian Mbappe is suffering from an acute case of gastroenteritis and has been admitted to hospital in order to undergo a series of tests and follow the appropriate course of treatment," said Los Blancos in a statement.
French striker Mbappe missed Real Madrid's Club World Cup opener against Al-Hilal on Wednesday, a 1-1 draw in Xabi Alonso's first game at the helm.
Alonso said on Tuesday the 26-year-old forward had been "feeling a little better" after illness in the run up to the match in Miami.
Madrid B-team player Gonzalo Garcia, 21, started in Mbappe's stead and opened the scoring for Real Madrid against their Saudi Arabian opponents.
Madrid's second match in Group H is on Sunday June 22 against Mexican side Pachuca in Charlotte.
Mbappe finished as the European Golden Shoe winner in his first season at Real Madrid with 31 goals in La Liga, but Los Blancos finished the season without a major trophy.
Find more details on group standings, match schedules, and competition statistics.
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