Real Madrid have announced that Kylian Mbappe has been hospitalised with acute gastroenteritis.
The France international is part of the Los Blancos squad that is currently participating at the Club World Cup in the United States.
However, the 26-year-old was forced to miss Wednesday's opening fixture against Al-Hilal, a game which ended in a 1-1 draw.
After the match, head coach Xabi Alonso suggested that the forward had already emerged as a doubt for their next fixture, saying: "I don’t know if Mbappe will be back for the next game.
"We’ll see. He wasn’t doing well in the last two days. He’s sick.”
Real Madrid release Mbappe statement
The La Liga giants are next in action when they square off against Pachuca on Sunday, that fixture commencing at 8pm (BST).
However, Mbappe has all but ruled out of that encounter after Real Madrid released a statement to confirm that the player had been admitted to hospital.
A statement read: "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."
As it stands, a potential recovery period has not been suggested, but Mbappe is realistically in line to miss the remainder of the group stages at the very least.
Major blow for Alonso
With Mbappe having scored 43 goals from 56 appearances in his first campaign at the Bernabeu, his absence represents a major blow for Alonso.
Gonzalo Garcia, a 21-year-old striker with just six senior appearances to his name, started in Mbappe's place on Wednesday but failed to make a notable impact.
Although the likes of Rodrygo could revert to a central role, Alonso is short of experience for the number nine role when Mbappe is missing.
The hope now will be that Real Madrid can negotiate games with Pachuca and Red Bull Salzburg to reach the last 16 and have Mbappe back in the squad for the knockout phase.