The Cubs called up the highly touted shortstop in an effort to give both the offense and defense a boost.
"We felt he could come up and we would get better defensively having him at short and moving Ryan [Theriot] to second, which we think will be good for him, too," Cubs general manager Jim Hendry said Friday. "We feel at this point in his career, [Theriot] might be better suited to go to the other side of the bag.
"It's no secret we haven't been swinging the bat on a consistent basis," Hendry said. "We've had a lot of good pitching and really two bad starts in the last 14, 15 games. ... We've had a real good run in the 'pen. We haven't knocked in a lot of runs when we could've won a lot of ballgames. Until we start swinging the bats better, defensively we'll be in better shape."
Castro, who turned 20 in March, was batting .376 in 26 games at Double-A Tennessee. He had one homer, eight doubles, five triples and 20 RBIs. Infielder Chad Tracy, who was batting .273 in 19 games, was optioned to Triple-A Iowa to make room on the roster.
This spring, the young infielder impressed the Cubs but team officials said they wanted to be patient with his development. In 2009, he batted .302 at Class A Daytona and .288 at Tennessee, then hit .376 in the Arizona Fall League.
Castro's arrival means Theriot shifts to second base, and the Cubs didn't waste any time. Just before 3 p.m. ET, Castro, Theriot and coaches Alan Trammell and Ivan DeJesus were working on the field at Great American Ball Park on positioning. Theriot made one of three Cubs errors in Thursday's 11-1 loss to the Pittsburgh Pirates.
"Nobody's a finished product when they get to the big leagues at 20," Hendry said. "We certainly don't want him to feel that kind of pressure. Sometimes a young player does give a club a lift. It wasn't a reaction of losing the third game last night (at Pittsburgh)."
Hendry said he had been considering promoting Castro since Monday.
"This move was going to come sometime this year," Hendry said. "That was no secret. All winter, all Spring Training, Ryan and I had a few talks about that, even before yesterday. [Theriot] knew this move was coming sometime. We felt we're going to do it now instead of waiting any longer."
On April 30, Hendry dismissed rumors that Castro's arrival to the big leagues was imminent, saying the shortstop had more to learn. Cubs assistant general manager Randy Bush had watched Castro play in late April, which sparked rumors of his promotion.
The move means Mike Fontenot and Jeff Baker will get less playing time. Manager Lou Piniella will have to figure out a way to use both infielders, who had been sharing second base.
The Cubs, who are batting .220 on the road and have been inconsistent on offense, are very right-handed again in the Major League lineup.
"Our better guys have to get going to start knocking in more runs," Hendry said.
After Bush's visit, Hendry said the club wanted to give Castro a little more time. That changed.
"We weren't winning a lot of games that we could have by knocking in runs," Hendry said. "Until we started hitting, we thought we'd go this way and put Ryan at a spot we thought he'd be better at defensively and try to play the best defense we can up the middle with Starlin."
Castro, who is 20 years 44 days old, will wear uniform No. 13. He's the youngest Cubs player to make his Major League debut since Oscar Gamble on Aug. 27, 1969, at the age of 19 years 250 days. In addition, Castro will be the youngest to make his debut with the Cubs as a shortstop, surpassing Marty Shay at 20 years 144 days on Sept. 16, 1916.
Since 1920, only 18 players have made their Major League debut as a Cub at a younger age than Castro. He will be the youngest ever in the franchise to make his debut at shortstop.
In other Minor League roster moves, the Cubs promoted pitcher Andrew Cashner from Tennessee to Iowa. The right-hander, who was the team's No. 1 Draft pick in 2008, was 3-1 with a 2.75 ERA in six starts. He had 42 strikeouts and had walked 13 over 36 innings.
Josh Vitters, the Cubs' top pick in 2007, was bumped up from Class A Daytona to Tennessee. The third baseman was hitting .291 in 28 games with three homers, eight doubles and 13 RBIs.
"These are marquee guys from our system," Hendry said. "Just the timing worked out where it was time to work them up at the same time as Starlin."
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.