The newspaper cites a tweet from Sandoval's brother Michael.
Sandoval was supposed to have been released on Wednesday, but doctors are keeping him for observation.
Sandoval is playing in a Venezuelan baseball league during the Giants' offseason. Team officials say he could be out as soon as Friday.
Sandoval was named to Venezuela's provisional World Baseball Classic roster on Thursday, along with second baseman Marco Scutaro.
Sandoval, was expected to be in New York this weekend to pick up the Babe Ruth Award as last postseason's Most Valuable Player, but his illness now makes it unclear if he will attend.
The San Jose Mercury News and MLB.com contributed to this report.
