Quade decided to keep Castro out of the starting lineup Monday, calling it a "mental day off," rather than a benching.
"I was really disappointed and surprised, very surprised," Quade said before the Cubs lost to the Braves 3-0. "But it's something from Day 1 focus-wise. It's something, to be honest with you, that he has to work on. With his talent, that's the toughest challenge for him.
"It's simply not acceptable, not good. He feels terrible. I talked to him today, and we are giving him a day. After seeing last night, you have to make his concentration a priority. Getting him to concentrate pitch to pitch. He can do it, and we think he will. And we'll make sure that happens."
Cameras showed the 21-year-old Castro, who was the Cubs' only All-Star this season, not paying attention and reaching into his pocket for sunflower seeds during pitches in the sixth inning. As reliever James Russell went into his motion and delivered to Daniel Descalso, Castro kicked at the dirt and then walked toward the outfield, never looking toward home plate when the pitch was thrown.
"When I saw Russ' pitch when he wasn't on board yet, that was the thing that got me more than anything," Quade said. "People stay loose different ways and approach each pitch different ways, but the fact that he wasn't prepared for Russ' pitch was the main thing."
Castro said he didn't even know he missed the play.
"After the game (Alfonso) Soriano said, 'Did you see that? You missed it,'" Castro said before Monday's game. "I said I didn't know but I saw it on TV, and it will never happen again. I said I'm sorry to my teammates and the coaching staff and everybody."
ESPN analyst Bobby Valentine, a former major league manager, pointed out Castro's abundant talent and potential, but a graphic during the telecast pointed out that Castro leads the National League in errors with 21. Valentine also referenced comments Cubs chairman Tom Ricketts made after firing general manager Jim Hendry recently.
"Tom Ricketts mentioned he wanted to see player development, and sometimes you have to develop them at the major league level," Valentine said during the broadcast. "One of the things you have to teach him is always to watch the ball. Wherever the ball is, you have to be looking.
"Castro's thinking about his at-bats or thinking about the play that was made, and something like that is absolutely inexcusable at the major league level."
Cardinals pitcher Jake Westbrook reached on an infield single to Castro earlier in the inning.
"This is a teaching moment where (third base coach) Ivan DeJesus came in and talked to him about the throw that he made on Westbrook," Valentine said. "He didn't talk to him about having his head down.
"Infielders can't be walking around with your head down. Infielders always have to know where the ball is and always have your head up. And what you need to do to motivate players is you need to reward them when they do well and you need to reprimand them when they don't. This was an instant that needed reprimand and I think it went unnoticed."
Castro said he has to pay the price for the gaffe.
"I wasn't tired mentally," Castro said. "But somebody has to pay for that, and I am paying for that. And that's why I'm not playing today."
Valentine said it was important for Quade to act quickly.
"When you're at a major league field and that happens, it must be addressed as soon as possible," Valentine said. "What (Castro) does that will aggravate people, and he needs to stop doing it, is he's looking down all the time. He wants to look down and think about things as he's walking.
"What he needs to have is awareness of everything around him. The ball, his teammates, the opposition, and not be thinking about what happened or what's going to happen but get into the moment."
At one point, Quade was asked if Castro was going to be tested for Attention Deficit Disorder.
"You spend enough time around him, no one has given that a consideration," Quade said. "Let's just ask Cassie to concentrate and do a better job, get the most out of his ability."
Castro is hitting .308 and recently became the youngest Cubs player in more than 70 years to reach 300 career hits. Quade said that while the organization will be diligent in its development of Castro, it's ultimately up to the player.
"It's on Starlin to make this progress," Quade said. "He's got a coaching staff that cares about him. He's got all the things in place to do this and do this well."
ESPNChicago.com's Bruce Levine and ESPNChicago.com contributor Chris Silva contributed to this story. Information from The Associated Press was used in this report.