A couple of ways to look at this question are to:
- Check whether history suggests that an extremely poor rookie fielding percentage such as Nunez' corresponds to the length of the player's career at shortstop
- Look at individual precedents.
Note: This post does not suggest that fielding percentage equals defensive ability. It merely measures one facet of defense - the fundamental ability to catch and throw.
To check this out, I looked at the rookie seasons for all shortstops since World War 2 who fielded at least 125 chances, to see if there is some correspondence between their rookie fielding percentages and the number of career chances they had after their rookie season.
The league average shortstop fielding percentage has risen in this period from .956 in 1946 to a high of .974 (in 1997, 2005, and 2008). To adjust for these differences across time, this analysis uses Fielding Percentage Plus (FP+), which is described in Diamond Mind Baseball's Explanation of Statistics page.
Notes: Diamond Mind Baseball calls this statistic "Fielding Average Plus"; for some reason, Diamond Mind Baseball uses the term "Fielding Average" instead of the traditional "Fielding Percentage". Also, since the MLB definition of "rookie" has changed across time, the older definition of rookie - 75 ABs - is used.
I posted a spreadsheet containing this data to Google Docs - see https://docs.google.com/spreadsheet/ccc?key=0AkMu391-e5t0dHJrY2tjQlVPd2NkVW1oNWNfbnFmTUE#gid=0. As you can see, the only rookie shortstops since World War 2 worse at catching and throwing are Juan Beniquez and former Yankee Erick Almonte.
What I wanted to see is whether rookies with extremely poor rookie fielding percentages tended to have shorter careers at the shortstop position; the goal is to find some basis of comparison for Nunez. To do this, I looked at the relation of rookie season FP+ and total career post-rookie chances at shortstop.
Since we are looking at career totals, I removed all shortstops who either had a rookie season after 2006 or is still listed as active in the Lahman database. Shortstops who started out the last few years would not have yet had an opportunity to have a full career, so including them could have skewed the data.
Here are some stats on the remaining 307 shortstops:
- Their median rookie FP+ was 99.3 - meaning the typical rookie shortstop had a fielding percentage around 99.3% of league average.
- The vast majority of the rookie shortstops had FP+ scores ranging from 97-103. In other words, most of the rookie shortstops were in a range of 3% worse to 3% better than league average.
- 21 of the remaining 22 shortstops had FP+ below 97; fielding percentages more than 3% worse than league average. The following histogram shows the negatively skewed distribution:
To give some context for post-rookie career chances, the median shortstop in this group had 852 post-rookie chances, and only around 25% of them had more than 3000 chances.
Now we can finally look at the important numbers. I divided the shortstops into quartiles based on their rookie season FP+, the shortstops with the lowest 25% of FP+ by far had the least career chances:
|Quartile #||FP+||Median Career Chances|
The lowest quartile includes some shortstops with extremely low FP+ scores, with the lowest being 93.1. If we break these guys into 2 groups, the 10% of the shortstops with the worst FP+ scores have 217 median post-rookie chances, while the shortstops in the 11th through 25th percentiles have a median 695.5 career such chances.
You might ask, what about MLB shortstops with rookie seasons prior to America's involvement in World War 2? I did the same analysis, and the results were similar. Throughout major league history, shortstops with extremely bad rookie fielding percentages are likely to have much shorter careers at the shortstop position.
That being said, a small subset of such shortstops do go on to field a significant number of chances at shortstop. The following table lists the shortstops in this study who had horrific rookie fielding percentages - this is the group in which Nunez would fall:
|Shortstop||Rookie Year||Rookie Age||Rookie Chances||Rookie FP%||Rookie SS MLB FP%||Rookie FP+||Post-Rookie Career Chances||Career FP+|
As you can see, the bulk of these guys had insignificant careers at shortstop. However, there are a few exceptions. Garry Templeton clearly is the most significant; however, since Templeton was only 20 years old as a rookie, he's not an apt precedent for Nunez.
And then some of these guys went on to have careers at other positions such as second base and outfield - for example, Felix Mantilla, Gary Sutherland, and Juan Beniquez.
In this latter respect, it's worth mentioning a player who is active and thus not in the above table, Oriole second baseman Brian Roberts. Roberts' minor league stats indicate he was groomed by the Orioles as a shortstop. However, after his 2001 rookie season, in which Roberts FP+ at shortstop was 96.7, the Orioles apparently sent him to the minors to learn second base.
Let's circle back to Nunez. The historical precedents obviously suggest the odds are against him evolving into a shortstop with competent hands and arms. Of course statistics are just a generalization; it's not impossible that he will learn to concentrate better, or to have better mechanics in the field. The Yankees must feel it is worth a shot, since when they demoted him to the minors they announced he would play shortstop and not switch to a less demanding position such as second base. But the precedents do suggest that Nunez would be a very rare bird if he ends up as a regular major league shortstop.