Castro had been slated in his customary cleanup spot for Wednesday's game against the Giants, an 8-3 loss, but was scratched before the first pitch.
"It's a family tragedy," Cubs manager Rick Renteria said Wednesday, "so we'll just take it one day at a time."
Players on the bereavement list are granted a minimum of three days and a maximum of seven away from the club.
The team has yet to make a roster adjustment to account for Castro's absence.
Renteria said that Javier Baez, who played the bulk of his games in the Minors at shortstop, would take Castro's post until he returned, as he did on Wednesday. Baez moved from second base, where he's played since being called up on Aug. 5. Utility infielder Chris Valaika took over at second.
Castro is the team's iron man, having missed just one start in 125 games before Wednesday. He has a slash line of .284/.333/.429, with 13 homers and 64 RBIs, and returned to the All-Star Game this year.
Daniel Kramer is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.