"I feel really bad about what happened," Castro said Tuesday. "I'm working too hard for that kind of thing to happen. It won't happen any more."
The mental error came in the Giants' fifth Monday. San Francisco trailed, 2-1, and loaded the bases with none out. Jeff Samardzija got Joaquin Arias to line out to second base. Brandon Crawford then hit a potential double-play ball to second baseman Darwin Barney, who flipped to Castro covering second.
But Castro apparently thought that was the third out and headed toward the dugout without a throw to first. A run scored on the play to tie the game. The Cubs eventually lost, 3-2.
"He felt awful about it," Sveum said Tuesday. "We talked about it and are trying to eliminate those kind of things happening. He knows that, and he knows he has to get better and [improve] the focus part of [his game]."
Castro, 22, made his Major League debut in May 2010, so he's now played two full seasons in the big leagues. He's one of the youngest players in Major League baseball, but that doesn't mean he gets a free pass.
"Was it a big thing? Of course it was," Sveum said of the gaffe. "The fact of the matter is, veterans do the same thing sometimes, so it's not just a case of the guy being young. Everybody on this team and everybody who is 22 years old still has played a lot of baseball.
"There are certain things that happen that people get embarrassed by. He felt horrible about it. You're not going to bench a guy for something like that, you're going to bench guys for not playing hard or not preparing and things like that. It's one of those unfortunate things that happen that you try to eliminate."
Sveum has worked with Castro since the first day of Spring Training, trying to cut down on the shortstop's errors. He's made two since April 21 after being charged with seven in the first 15 games.
"People forget, he's come a long way defensively and hasn't made a fielding error in six weeks," Sveum said. "We're forgetting about the big picture -- the guy is still hitting .300, his defense has come a long way -- everything has come a long way as far as I'm concerned from looking from the other side of the fence."
Sveum emphasized the importance of the position and how part of the shortstop's job is to make the other players around him better.
"I think he knows that, and the youth sometimes holds that back," Sveum said. "It's a matter of making him understand he has to make people around him better, and the only way you do that is by work ethic and trying to be a leader on the field."