Castro has started all of the Cubs' 82 games, including Thursday's series finale against the Braves. His only other challenger was Atlanta's Dan Uggla, who had Saturday off after starting the first 75 games of the season.
Castro, though, is 3-for-21 (.143) in his last six games, and entered Thursday's contest batting .291, a season low. His glove is keeping him in the lineup.
"That defense is saving us a lot of runs right now," Sveum said.
Castro, 22, who will be making his second All-Star appearance on Tuesday in Kansas City, never asks for a day off.
"You start seeing it in people's faces more than their performance," Sveum said. "When you have a defense that, knock on wood, is playing as well as ours right now, you don't want to do anything because of offensive [struggles]. We're playing too good now to think about that."