Starlin Castro has not been a good defensive shortstop to this point in this career. His career UZR is -11, his DRS is -15 and his Total Zone (TZ) is -27. His 60 errors since 2010 leads all MLB players. Ian Desmond is close at 58, but has played a little more than Castro. Those two are nowhere close to the next most errors of 47 (Cliff Pennington). Those 3, along with Mark Reynolds (46) and Elvis Andrus (41) are the only players with 40 or more.
Ian Desmond leads the league in fielding errors with 41 and Castro is 2nd with 30. Castro's 30 throwing errors is 4 higher than Alcides Escobar's. Castro makes especially poor throws and is an equally poor fielder.
One way to look at how Castro may perform defensively as he ages is to compare him to other similar players. The problem is that there aren't many similar players. In fact, there have only been 7 other shortstops (75% of games played at SS) since 1947 (integration) who have had 800 or more plate appearances through age 21. Those 7 are Elvis Andrus, Edgar Renteria, Alex Rodriguez, Jim Fregosi, Garry Templeton, Alan Trammell and Robin Yount. Needless to say, Castro is a rare player. It's obvious you just don't see shortstops at his age with the kind of experience he already has.
Among these 8 players, Castro's TZ is worst at -27. Robin Yount is just ahead of him at -26. Then you have Trammel (-9), Templeton (-7), Fregosi (-5), A-Rod (-1), Renteria (1), and Andrus (6). It also becomes clear by looking at this that young shortstops just aren't very good defensively. The 8 have a combined TZ of -66. Obviously this group didn't force their way on the MLB roster because of their defensive skills. For what it's worth, they also weren't especially good hitters other than A-Rod. A-Rod was 62 batting runs above average, Templeton 13, Fregosi 8, Castro 6, Trammel -20, Renteria -26, Andrus -31 and Yount -45. I'm not concerned with their offense right now. Castro isn't going to be A-Rod, but who thought he would be?
I'm wondering how the others did after their age 21 season. Andrus has played just one year beyond that and was worth 9 TZ. We'll throw him out since he only has that season.
Renteria was worth +5 over the 3 seasons following his age 21 season. He showed only modest improvement. A-Rod was worth 19, but to be fair, that includes a +16 season. Fregosi's TZ was 31. Templeton's was 12. Trammell's was 27. Yount was worth 22.
Yount gives us the biggest reason to be optimistic about Castro's defense. His improvement was considerable, but it's also important to point out that Yount's -26 through age 21 included twice as much playing time as Castro has had. Castro is coming from a point that we really haven't seen before, but there have been so few SS with this much playing time that it's hard to estimate what he may do in the near future.
It's clear, as well as obvious, that these players improved defensively. It's not clear by how much. Renteria barely improved. Yount, Trammell and Fregosi improved by a considerable amount. I can't help but notice Garry Templeton's name again.
Castro performed offensively almost exactly the same as Templeton did in High A and in AA at the ages of 18 and 19 respectively. Castro returned to AA at the age of 20 and hit the shit out of the ball while Templeton was assigned to AAA and hit very well there. Castro had about 300 fewer plate appearances at the age of 20 in the minors than Templeton, but throughout the minors these two were very similar to one another.
Their first two seasons in the big leagues?
Castro's OPS+ was 107, Templeton's 106. Templeton had 5 rWAR and Castro had 2.8 rWAR. The difference was the defense, which is listed above. Templeton had 19 batting runs above average to Castro's 6 and Templeton did it 350 fewer plate appearances. Templeton had an 85 OPS+ over the rest of his career and 20 rWAR. Let's hope that in 10 years we look at these two players and the only thing they have in common are the final 2+ seasons in the minors and first 2 seasons in the big leagues.
As for Castro's defense in the future? Your guess is as good as mine. He'll improve, but more than likely will never be better than average even if he improves as much as the others who improved the most. If he improves like Renteria did, Castro won't be playing SS much longer.