"I think sometimes you end up looking in somebody's eyes to see when [the time is right]," said Sveum, who has been considering the move for a few weeks. "I don't like to give any core players a day off at home. When people pay a lot of money to see their favorite player, you don't want to do that at home."
The Cubs had Monday off, and Tuesday was the start of a three-city, nine-game swing.
Sveum said he felt Castro needed a mental break.
"I told him to think about tomorrow as a whole [new] season and not try to hit .300 tomorrow," Sveum said. "It's not going to happen. You can't get four hits every at-bat and think you're going to get your average back up there. There's plenty of time left in the season to hit .330 and you can finish at .290 or .300.
"I told him, 'You're not the first really, really good hitter to be in these kind of slumps. It's part of everyone's career.'"
Castro had the longest active consecutive games played streak in the National League at 269, second in the Majors to Detroit's Prince Fielder, who has played in 418 straight.
"He's such a talented player who's struggling, and I think he's never struggled like this before," Cubs general manager Jed Hoyer said. "He's probably somewhat physically tired, and definitely frustrated with his performanceb and hopefully this will help him clear his head."
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.