There were 20 players at the tryout in the Dominican Republic that day in 2006. Castro, who was then 16, had been recommended by a part-time scout to Serra, who is the Cubs' Latin American scouting coordinator.
"He was the best player in the whole tryout," Serra said. "Thank God we got him."
It took some negotiations. The Cubs offered $35,000, and Castro's camp asked for $60,000. They settled on $45,000. The shortstop played in the Dominican Summer League in 2007 and hit .299. He hit .311 the next year for the Mesa Rookie League team, then .299 combined at Class A Advanced Daytona and Double-A Tennessee.
"He hasn't stopped hitting," said Serra, who was smiling like a proud father during a stop at Wrigley Field on Tuesday.
Castro, 21, has kept up the pace in the big leagues, and he heads into the Cubs' final six games one hit shy of becoming the youngest player in franchise history to total 200 hits in a single season.
"I'm very surprised," Serra said. "I thought he'd be a big leaguer, but I didn't think it would happen this fast. Being as good as he is at this moment, you think a Dominican kid -- comes from a poor family, an uneducated family -- he's got some talent, but you think it will take eight years to figure it out.
"This guy, it didn't take any time," Serra said. "Here he is, almost 200 hits."
Castro tried to reach the milestone on Wednesday in the Cubs' last home game, against the Brewers, but he came up short, going 2-for-3 with two walks, including an intentional pass. Castro had one more chance in the eighth and the crowd was on its feet cheering, but he drew a walk. Castro was ahead in the count, 3-0, but did not have the green light to swing. Chicago had a six-run lead at that point.
"It's just not proper, and it's not good baseball," Cubs first baseman Carlos Pena said about the option of swinging at a 3-0 pitch. "It's an unwritten rule; it's etiquette. It's the best pitch he probably saw all day. I was hoping the 3-1 [pitch] was close, because he was ready.
"We're all anxious to see it happen," said Pena, whose personal single-season high is 138 hits, achieved in 2007. "It's a very impressive feat to accomplish. I'm just in awe and impressed with how well he's performed."
Castro isn't just hitting, either. He's reached safely in 34 straight games, which ties the longest stretch by a Cubs shortstop, set by Woody English in 1929. Castro is the first Cubs player to reach safely in 34 consecutive games since Jerome Walton had a 43-game streak in '89.
Castro is on the verge of reaching 200 hits at a younger age than former Cubs Billy Herman, who had 206 hits in 1932, and Augie Galan, who had 203 hits in '35. Both were 23 at the time.
In the last 71 years, starting in 1940, only Alex Rodriguez, Garry Templeton, Vada Pinson and Al Kaline have totaled 200 hits in a single season at 21 years of age or younger. Rodriguez totaled 215 hits in 1995 with the Mariners; Templeton had 200 in '77 with the Cardinals; Pinson had 205 in '59 with the Reds; and Kaline had 200 in '55 with the Tigers. Kaline is the youngest, having reached the milestone when he was 20; the others were 21.
Castro has primarily been the Cubs' leadoff man this season, although he did sub in the No. 3 spot when Marlon Byrd was injured. Castro is batting .326 when batting first, .335 when hitting second and .224 at No. 3. He's shown more power this season, hitting 10 home runs in 649 at-bats after hitting three homers in 463 at-bats last season, his rookie year.
|"I'm just in awe and impressed with how well he's performed."|
|-- Carlos Pena, on Starlin Castro|
Where does Serra see the youngster batting?
"I think in the future, not too far from now, he could hit third," Serra said.
Where does Castro want to hit?
"Second," Castro said, without hesitation. "I feel very good [batting] second."
That's a topic for next season. Right now, the Cubs have six games remaining, and Castro will pick up his pursuit of No. 200 on Friday in St. Louis.
"I'll be ready for Friday in St. Louis," Castro said.
After his first season in the big leagues, Castro returned home to Monte Cristi, a fishing town on the northern coast of the Dominican, and bought his family a house. His father no longer has to work. Castro's family was with him in Chicago for much of the season, so he could take advantage of his mother's home cooking.
Serra smiles whenever he talks about Castro. The future is bright. The scout isn't the only one who has noticed.
"I just love the fact that inside, he has this inner confidence and knows how good he is, but he's also humble enough to understand it takes work," Pena said. "He appreciates the game and respects it and knows how to bow his head when he has to. That will be very valuable to his career.
"I can't wait until he gets to 200," Pena said, "because it'll be a special moment for all of us."
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.