This year, he's had 570 at-bats in 153 games, swiped 37 bases, totaled 178 hits, including 28 doubles, seven triples, four homers, and batted .312. Nothing has fazed him.
"He's very talented and really on some kind of fast track," Cubs player development director Oneri Fleita said of Castro. "Quite frankly, we haven't had a player go through the organization like this in a long time. I think the last one was Mark Prior, and pitchers are a little different."
Could Castro be in the big leagues as soon as 2010?
"Me, personally, I'd like to let the kid be a kid and let him go to Spring Training," Fleita said when asked to project where the shortstop will play next year. "When he's ready to go, he'll tell us. We'll know what we've got."
The Cubs do have shortstop Darwin Barney in line to start at Triple-A Iowa, and Castro could open at Double-A Tennessee. However, Fleita would like Castro to at least take part in the Cubs' big league camp in February so Lou Piniella and the coaching staff can get to know him.
"At this point, to say he'll be in Double-A or Triple-A at the start of the season, I think that's premature," Fleita said. "We'll take our time. We know we have a diamond in the rough and we need to take our time, be patient. At the same time, we don't need to hold the reins on him. When the time comes, turn him loose and let him fly."
Ryan Theriot is not about to hand Castro the job. Theriot is coming off a season in which he batted .284 and made 147 starts, most by a Cubs player at that position since Shawon Dunston started 150 games in 1988. Castro did play second base in the AFL, but that's not because the team wants him to switch. Fleita said they'd agreed to moving Castro to accomodate Boston Red Sox prospect Jose Iglesias, who was also on the Solar Sox roster.
"It's expected from all infielders to play all infield positions so they understand the bunt coverage and what their teammates expect on the other side," Fleita said. "We don't see that as a problem if he played second. I just wanted to get his at-bats and get him experience at that caliber of play."
Which Castro did. The teenager did lead the AFL in batting for a large part of the season.
This year, he ranked third in the Florida State League with a .302 average, and recovered from a slow start, hitting .324 from May 1 to the end of the season. He was named to the FSL All-Star team, and won MVP honors, going 4-for-4 with an inside the park home run. In 31 regular season games at Tennessee, Castro batted .288.
This was just his third season of pro ball. He doesn't turn 20 until March 24. Baseball America ranked Castro as the Cubs' No. 1 prospect, noting he could be the team's first All-Star shortstop since Dunston in 1990.
He's a slender 190 pounds, and his defensive play reminds some of other Dominican shortstops such as Tony Fernandez and Miguel Tejada.
"I think the thing I'm most proud of is he's been able to maintain his weight," Fleita said. "You go to the Florida State League and it's blistering hot and humid, and to be able to play daily like he did, sometimes a young kid can wither away. Then to go to the Fall League, you'd expect there would be a little bit of a lull but he's been able to maintain his strength and his level of play.
"I'm really proud of him," Fleita said. "It's been a real joy to watch his progress throughout the year."
As for the Cubs' other entries in the Arizona Fall League, pitcher James Russell posted impressive numbers. A 14th-round pick in 2007, Russell gave up two earned runs on 12 hits and two walks over 14 1/3 innings for a 1.26 ERA. He struck out 14.
Lefty John Gaub, acquired from Cleveland in the Mark DeRosa deal, posted a 9.31 ERA in 10 games for Mesa. He did strike out 15 over 9 2/3 innings. Blake Parker, a 16th-round selection in the 2006 Draft, struck out 11 and walked six in 12 innings over 11 games, giving up six runs for a 4.50 ERA.
Their futures are just as undetermined as Castro's. What about next year? Cubs fans will have to wait and see where the shortstop ends up.
"Things will take care of themselves," Fleita said.