The Cubs made the announcement before Wednesday night's game at Wrigley Field against the San Francisco Giants. The team didn't disclose specifics on the emergency or how long Castro might be out.
Castro has played in all 125 games for the Cubs games this season and is batting .283 with 13 homers and 64 RBIs. He had been set to bat fourth on Wednesday.
Rookie Javier Baez moved from second base to shortstop to take Castro's spot on the field. Luis Valbuena was inserted in the cleanup spot in the order.