Castro, at the tender age of 21, has seemingly erased any doubt about a possible "sophomore slump" in his second year in the Windy City.
The question the Cubs must be asking is, "How high is his ceiling?"
Players as young as Castro generally don't sniff a major league ballpark until September, when rosters are expanded and teams do unofficial "tryouts" for next season.
But Castro has already logged over 550 at-bats as a big-leaguer—and he's looked really good in most of them.
His defense remains a work-in-progress (he committed three errors in Monday's game against the Colorado Rockies), but he is young and athletic enough to develop into a good defensive shortstop.
But offensively, there is something special here.
It is still early in the 2011 season (so forgive me for gushing), but Castro shows glimpses everyday of what could be in store for the Cubs for the next dozen years.
Currently sporting a robust .350 batting average while swiping four bases and scoring 17 runs, Castro came out of the gate very strong in 2011.
His .361 batting average on balls in play will almost certainly come down to a more normal level, but Castro should still be able to hit around .320 or so.
His line-drive percentage currently sits at 22.4 percent—fellow slugging shortstops Hanley Ramirez and Troy Tulowitzki have never posted a line-drive percentage as high.
Again, it's a bit early to be projecting Castro's final numbers, but this is a kid who hit 31 doubles a year ago in just 125 games—so we know the bat is capable.
After the Cubs are done asking how high Castro's ceiling might be (though they and us probably won't know that for a while) they'll want to ask themselves where Castro should bat in this Cubs lineup.
Early on in his career, Castro has primarily batted No. 1 or No. 2 in a Cubs uniform.
It's easy to see the speed and athleticism and the obvious havoc he can cause on the base paths—both good reasons to bat him at the leadoff spot.
But Castro hasn't exactly been a base-running menace in the early parts of his career.
He stole just 10 bases a year ago (though he attempted 18 steals), and never logged more than 28 in his days as a minor leaguer.
What's worse is Castro's patience at the plate, or lack thereof.
Early on, he's only drawn four walks. He only sees 3.6 pitches-per-plate-appearance (only two Cub regulars see less).
It's hard to justify batting Castro leadoff when he sees so little of the opponent's starting pitcher.
The free-swinging ways obviously work for Castro, but they are not attractive traits for a leadoff man, who is supposed to work counts and show his teammates some of the opponent's stuff.
His .374 on-base percentage is heavily aided by his .350 batting average, so we must look past that number.
Castro could very well become the team's No. 3 hitter and the lineup would not skip a beat—it could actually improve some, in fact (think about plugging a .350 hitter into the No. 3 slot).
The problem with moving Castro, is that there's no problem at all.
Let me explain.
Castro in the leadoff spot is hitting an other-worldly .436 in his career (this comes in just 55 at-bats, however). His OPS (on-base plus slugging percentage) is a ridiculous 1.074.
Can you really take someone out of a spot they feel so comfortable in?
That's the question Cubs manager Mike Quade needs to be asking himself.
I'll go to Florida Marlins superstar shortstop Hanley Ramirez, who has essentially only batted in two places in his career as a Marlin, leadoff man (1,631 at-bats) and the No. 3 spot (1,311 at-bats).
I'm not saying Castro is the next Ramirez, I'm simply looking at what has worked best for Ramirez because I feel Castro, if he adopts a more-patient approach, could someday rival Ramirez.
Basically, Hanley Ramirez has been a terror wherever he's batted.
He posts a .387 career on-base percentage in the leadoff spot, with a .538 slugging percentage.
As a No. 3 hitter, Hanley goes to a .382 on-base percentage, but sees his slugging percentage dip a bit to .487—nothing to scoff at, but noticeably less than his slugging percentage at the top.
So, as a leadoff man, Ramirez sports a very solid .925 OPS, making him one of the game's most dangerous lead-off hitters.
As a No. 3 hitter, Ramirez shows a .869 OPS—again, very good, but significantly less than the leadoff spot.
As the numbers suggest, Ramirez has been better batting at the top, and the numbers for Castro support that as well.
In the small sample size I mentioned above, Castro has a .456 on-base percentage and a .618 slugging percentage, making the 1.074 OPS.
As a No. 2 hitter, however, Castro is very average.
His slugging percentage dips below .400, and his OPS plummets to .734.
The guess here is that Castro, despite his swing-first, look-second mentality, is more comfortable batting atop the Cubs' lineup—and the Cubs need to appreciate that fact and keep Starlin at No. 1.
With such a drastic set of numbers staring them in the face, the Cubs should anoint Castro as their team's leadoff man for the time being.
With a larger sample size, of course, Castro's numbers from the leadoff role figure to change, but that's an issue for another day.
Kosuke Fukudome is also a very capable leadoff man for the Cubs, but his time in Chicago is brief and Castro should get the vast majority of leadoff man at-bats as long as his numbers from that spot stay strong.
He is young enough to learn how to hit from different spots, of course, but in the early part of his career, Castro looks like he could become the next great leadoff man in the National League—despite his low number of walks and pitches seen per at-bat.
If Castro were entrenched in the No. 1 spot, you would simply have to fill in the rest behind him.
We may not know how high his ceiling is, but we know that right now, it's the highest when he bats first for the Cubs.