"He was very upset," Manager Rick Renteria said Thursday.
One of Castro's relatives and three friends were killed in a car accident in the Dominican Republic on Wednesday.
Rookie Javier Baez will start at shortstop while Castro is away. Players on the bereavement list can miss from three games to seven games.
"My plan is just to keep [Baez] there until Starlin gets back," Renteria said. "It makes the most sense ... because that's what [Baez] has been doing his whole career. We just hope Starlin is OK."
The Cubs recalled infielder Logan Watkins from Triple-A Iowa to take Castro's spot on the roster.
Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter @CarrieMuskat. This story was not subject to the approval of Major League Baseball or its clubs.