Today we mark a milestone in the career of Jean Segura: it was 1 year ago today that he made his debut with the Brewers. Last August 6th, at Cincinnati, Segura got the start at shortstop and went 0-4. He started all 3 games of the series and only collected one hit in the finale. He went on to finish out the year as the primary starting shortstop.
As we look back at Segura's first year in the big leagues, it may be helpful to know something about 23 year-olds, and to know something about shortstops.
First of all, shortstops are not very good hitters. In the major leagues last season they were in fact the worst hitting position, excluding pitchers-- they had the worst on-base percentage and the worst slugging percentage. As a group they batted 20367 times and hit .257/.310/.378. Baseball fans intuitively understand this, but a shortstop who is a league average hitter overall (in 2012 that was .255/.319/.405) is a shortstop who is a much better hitter than most other shortstops and is accordingly more valuable than a league-average hitter at any other position on the diamond.
23-year olds are also, as a rule, not very good hitters. The most detailed league age batting splits we have available are Baseball-Reference's, and they break down groups between 25 and under, 26-30, 31-35, and 36+. The 25 and under group hit .252/.313/.396 last year, which is to be expected from a group not yet in its prime years in the baseball sense-- those 3 numbers were also the lowest in each component of the slash line among any of the age split groups. Do not let the once in a generation influx of outliers like Mike Trout and Bryce Harper cloud judgment too much, it is extremely uncommon for 23-year olds to move to the top of their profession when general consensus is that a baseball player's prime falls in their late 20s and even early 30s.
The first part relates to how valuable Jean Segura already is. As a well above-average hitter and non-disaster defender, the answer is very valuable. The second part relates to how much better he could become as he gets closer to his prime years. The answer to that is that we have no idea how good he can be, but the fact that he has kept this performance up over a full year means that an even better year is within the realm of possibility.
It's an open question as to who might be the best shortstop in baseball right now with Troy Tulowitzki injured (it seems pretty clear he's an obvious choice when on the field). Here are a few candidates over the past 365 days, since Segura's debut with the Brewers, and their plate appearances and slash lines for the sake of comparison. Note that I am not dismissing the huge importance that relative defensive skill has in this discussion, feel free to browse Fangraphs and check out each player's UZR as I am not comfortable enough with each's defensive ability to offer any analysis beyond those numbers.
Jean Segura: 621 plate appearances, .300/.342/.434
Troy Tulowitzki, 328 PA, .321/.387/.590
Ian Desmond: 611 PA, .284/.340/.487
Jhonny Peralta: 641 PA, .271/.328/.417
Andrelton Simmons, 532 PA, .250/.293/.366
Elvis Andrus: 717 PA, .258/.313/.307
Jimmy Rollins: 693 PA, .256/.320/.389
Also of note:
JJ Hardy: 685 PA, .255/.294/.417
Alcides Escobar: 667 PA, .245/.276/.315
I'll stack up Segura's performance at his job against any other shortstop since the day he was called up. That is doubly encouraging, because not only does it mean the Brewers have one of the top shortstops in baseball, it means that there is still room for him to grow. He can round out the power and keep improving on defense. Look at some of the names listed above and remember them if someone tries to lump a bunch a group of young shortstops together-- Simmons, Andrus, Escobar, the field-- and realize that Mean Jean out hit them all in the last 162, his first 162, and he's just getting started.