Shortstop Starlin Castro was the only Cubs player represented on MLB.com's Top 50 Prospects list. That's OK. He's used to being special.

The 19-year-old has charged through the Minor League ranks since he signed out of the Dominican Republic in 2006, moving from the Dominican Summer League in '07 to the Rookie Arizona League in '08 to Double-A in '09 and then to the Arizona Fall League, where Castro was the prospect-laden AFL's youngest competitor.

Now he's the 22nd-best prospect in the game, according to the scouts and executives who helped MLB.com Minor League analyst Jonathan Mayo compile his list. Starlin's meteoric rise has Cubs fans wondering how long it'll be before he's taking grounders at Wrigley Field.

"I haven't had a player go through the system who has done what he's done," Cubs player development director Oneri Fleita told MLB.com in October. "I think we would've been happy if he hit .250 in [Class-A] Daytona. He's certainly surpassed any expectations we've had. To finish at Double-A and get a chance to play in the playoffs -- and he contributed to them winning.

"It seems like it takes some guys a year before we see any gains. We're seeing gains in Castro weekly, monthly, if not daily. Who knows?"

Castro ended up finishing sixth in the Arizona Fall League with a .376 average, and the rest of baseball has taken note. MLB.com contributor Peter Gammons said the Tigers asked for Castro and two others from the Cubs when the teams were talking about a trade for center fielder Curtis Granderson. Chicago officials balked.

Dave Keller, the Cubs' Minor League hitting coordinator, projects Castro will be with the big league club by the middle of the 2010 season. How does he know? One of the projects Keller assigns to the Minor Leaguers is to keep a journal. He doesn't want to read that they went 2-for-4 that day, but Keller asks that they describe their feelings, either at the plate or in the field or on the bench. The journals must be handwritten. Castro, who wrote his in Spanish, was organized and to the point, Keller said.

"He's a very confident kid, but not arrogant," Keller said. "For him, it's just a matter of getting repetitions."

Defensively, Castro may be big league-caliber already.

"He can definitely play shortstop at the big league level," said Josh Vitters, the Cubs' No. 1 Draft pick in 2007 and Castro's AFL teammate last year. "I can see how people could say that [Castro will be in the big leagues in 2010]. What he has in the infield, you can't really teach."

And from pitcher Andrew Cashner, the Cubs' No. 1 pick in the 2008 Draft and another AFL teammate: "Defensively, I think he's ready. He can hit. He's still a little young and he has a little more growing into his body to do, and he'll get more power. He's a great kid, a great player."

Ask Castro, and he'll tell you when he'll get to the Majors.

"Next year," the Cubs' future shortstop said in English.