Part of the rationale is to keep Castro focused on the game and avoid any mental lapses. Renteria said Castro, 24, who was promoted from Double-A in 2010, is still developing.
"He got here when he was 20 [years old] and we assume he knows exactly everything he's doing, and I think he's still learning," Renteria said. "I think we make a mistake that any player in the big leagues is not still learning."
Castro entered Monday's game batting .395 at home, and has hit safely in all nine home games.
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.