"We all know what the potential of this kid is," Cubs first baseman Carlos Pena said. "You may be seeing one of the best up-and-coming shortstops in the game. I'm just happy I have a good seat to watch him play."
The youngest player in the Major Leagues at 21 years and 14 days old, Castro also is the first under the age of 22 with at least 10 hits in the team's first five games. According to Fangraphs, he had not swung and missed a strike until the third inning Wednesday, when he struck out swinging.
The Cubs were more concerned about his glove work than his hitting, and preparation for the 2011 season began last November, when infield coach Ivan DeJesus went to the Dominican Republic for one week. At the plate, Castro was dazzling during his rookie season. He hit a three-run homer in his first at-bat May 7 in Cincinnati and finished with a .300 average. But he also made 27 errors in 123 games at shortstop.
The focus of DeJesus' lessons? Footwork.
"When he came up last season, his footwork wasn't in the right place, and that's what made his throwing angle bad," DeJesus said. "He didn't have his front side to the place where he was going to throw the ball. You correct your base and everything else will be right."
DeJesus and Castro would meet every day at the Cubs' academy in Boca Chica to do drills. Creating a solid base, a good foundation would help the shortstop with his balance and his throws. It wasn't hard to get Castro to work.
"He knew that for him to be the best shortstop defensively that he could be, he needed to improve on those little things," DeJesus said. "We did a lot of sessions with footwork more than his throwing. We progressed into game situations."
Castro played for Escogido in the Dominican Winter League, where he could apply DeJesus' lessons. He also got advice from some of the veteran players on the team. DeJesus returned to the Dominican for another week-long session before Spring Training. The message was still to focus on footwork, as well as to be prepared for more hard work in Mesa, Ariz. This spring, the coach and shortstop would meet early at least three times a week.
"What we tried to get into him is that when he does it hard in practice, it'll become automatic," DeJesus said. "We did a lot of situations in practice so he could concentrate better and do it in the game."
On the first play on Opening Day, Pittsburgh's Jose Tabata smacked a ball toward center. Pena figured it was a hit. But Castro raced over and somehow threw in time to get Tabata.
"Castro going up the middle, I don't know how many shortstops in the game could ever get to that ball," Pena said. "That was very impressive."
"The first play of the game, you go, 'Oh, my goodness,'" Cubs manager Mike Quade said.
On Sunday against the Pirates, the Cubs led, 4-3, going into the ninth. Closer Carlos Marmol was on the mound, vying for his second save. Pittsburgh had runners at second and third with one out when Pedro Alvarez dribbled the ball to Marmol's right. The pitcher missed it, but Castro scooped up the ball and threw to Pena, who somehow stopped it up the line. Two runs scored on heads-up baserunning, and the Cubs lost.
Looking back, Castro probably should've held onto the ball and not tried to make a play.
"His reaction is good," DeJesus said. "He reads the ball real quick. I asked about the [play], and he thought that Marmol was going to catch the ball, so he started a little late."
On Tuesday against the D-backs, the Cubs had a 6-4 lead going into the ninth. Arizona had runners at first and third, and pinch-hitter Xavier Nady hit a ball that deflected off pitcher Sean Marshall. It was almost identical to Sunday's play, but this time Castro reacted quicker, snared the ball and threw Nady out. The Cubs won, 6-5.
"He was the player of the game for them, as far as I'm concerned," Arizona manager Kirk Gibson said of Castro.
"The second one [against the D-backs], I think he reacted to the ball as soon as it was hit," DeJesus said. "That's one of the thing we talked about. He got in better position on the second ball than he did on the first one."
He may be young -- Castro turned 21 on March 24 -- and there will be growing pains. But he's matured quickly.
"Even your conversations with him, his receptiveness and his understanding of things now is so much better," Quade said. "He's just a young kid and he's learning. The fact that he understands and has taken responsibility for his play, too, that matters. That's how you get better and that's how you figure things out."
Said Cubs center fielder Marlon Byrd: "That aggressiveness is what we want. We don't want him passive. He's learning and getting better every game. His at-bats and defense, he's a superstar in the making."
DeJesus isn't the only one working with Castro. Aramis Ramirez was 19 when he made his Major League debut on May 26, 1998, with the Pirates. Now the Cubs' third baseman, he also keeps an eye on the young shortstop.
"I told him early this spring that last year, he made a lot of mental mistakes, and he made a few early in spring," Ramirez said. "He has all the ability in the world. He just has to concentrate and let his ability take over, and he's been doing that so far.
"He's only 21, he's going to make mistakes," Ramirez said. "He's a smart kid, he learns quick and he makes adjustments. I also made it to the big leagues when I was real young, and I made a lot of mistakes. I was 19; I've been in his shoes."
DeJesus knows, too. He was 21 when he made his Major League debut on Sept. 13, 1974, for the Dodgers. What helps Castro is he has a good foundation at home. His parents were with him in Spring Training, and they will be back in Chicago for the season.
"The support they can give him is important, especially in Chicago, when a lot of things could happen after the games, a lot of distractions," DeJesus said. "He has his mindset: 'Don't look to the left, don't look to the right,' and concentrate on what he has to do."
DeJesus is a tough taskmaster. If he sees Castro goofing around in batting practice or making lazy throws, he'll correct the shortstop. The goal is to practice the way he will play in a game.
Told that Ramirez also is keeping an eye on Castro, DeJesus smiled.
"That's nice when it comes from a player that he knows," DeJesus said. "It has more value for him and it's not me telling him all the time."
Castro also is humble. Alan Trammell was the Cubs' infield coach last season, and now is the D-backs' bench coach. Early on Monday, they saw each other and Castro made a point of thanking Trammell for his help.
"I will follow him the rest of his career," Trammell said. "That's how much he means to me."
Seeing Castro develop so quickly does make DeJesus feel proud.
"It's paying back everything I learned when I was that age, too," DeJesus said. "It stays with you all your life. That's what we're here for."
The two will be out early during the next road trip. There's still more work to do.
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.