Jhonny Peralta and a potential defensive decline

Jhonny Peralta's arrival in St. Louis resulted in many immediate questions. Some regarding his inconsistent offensive play. Many involving his suspension for PED use. On the field, the most important question Peralta will have to answer revolve around his ability to play shortstop. Offensively, anything Peralta does will be an improvement over .226/.282/.314 with a .260 wOBA and 63 wRC+. Defensively, Pete Kozma played a solid shortstop, 6.7 runs above average according to UZR. Peralta will receive over 30 million dollars over the next two years because he is expected to remain at shortstop. The Cardinals have the rest of the infield positions covered, and his bat does not profile at an outfield corner. With the Cardinals' ace, Adam Wainwright, a groundball pitcher, Peralta figures to receive a lot of work. Turning those chances into outs will help ensure Peralta earns his salary over the next couple years.

Defensive metrics have been generally positive for Peralta over the years. Peralta did not fare well with UZR during his first three full years at shortstop. Through 2007, he was nearly 30 runs below average according to UZR. Since 2007, however, he was performed much better, never being more than a run below average. The past three seasons he has averaged more than eight runs above the average shortstop in UZR.

There have been some questions about Peralta's range, and the Inside Edge statistics available at Fangraphs appear to back up that worry. Inside Edge employs scouts that examine every fielding play and move them into buckets depending on how easy or hard the play was to handle for the player. While the data is somewhat subjective, it does provide some insight into the types of plays Peralta is making. In 2012-2013, Peralta was given 42 chances on balls hit into the two most difficult buckets, 0% chance of making the play and 1-10% chance of making the play. While not necessarily an indictment of his range given the difficulty of those plays, Peralta did not field a single chance in those buckets. The next bucket, unlikely plays at 10-40%, Peralta received 27 chances. He made the play just once. Moving down difficulty levels helps some. At the 40-60% level, Peralta converted 36.4% of chances. When watching Peralta out in the field, do not expect the spectacular.

Where Peralta makes up for his lack of range is on the easier plays. Sticking with Inside Edge, on the 60-90% balls, he gets to a solid 82.4% of plays. Move down the scale to the routine plays and Peralta fields 98.5% of these balls. Given that there are so many more routine balls that Peralta does so well on, his defensive numbers remain strong. Peralta will have to make the adjustment of a new team with new pitchers, coaches and teammates in the field. An adjustment will be necessary, but based on history it is unlikely to affect his overall fielding numbers.

Looking at shortstops from 2005-2013 aged between 29 and 34, there have been 41 sets of seasons where a player qualified at shortstop in consecutive seasons without changing teams during the season. Of those 41 sets, nine times a player switched teams in the offseason. Because single season fielding runs can be misleading, the three year average before changing teams was used (fielding runs above average data from Fangraphs). Given that zero is average, the players represented a good cross-section of shortstops. The average of the same-team players had a three-year average of 0.13. The changing-team players were slightly worse, -0.22, but were very close to average as well.

As one-year defensive numbers are not as reliable without surrounding years, instead of comparing to solely the following year's numbers, the year after was added as well. While this may blunt the effect of determining an immediate change among the players that changed teams, the small number of players and the slightly less than reliable full year numbers would be of little use in trying to determine if players were harmed by switching teams. Over the following two years, the same-team group increased by 0.94 runs to an average of 1.07 fielding runs above average compared to an increase of 0.41 runs for the change-team group. A half-run difference is not nothing, but if Derek Jeter's miraculous two-year run as an average shortstop at the end of the previous decade is removed from the same-team group, the increase drops to 0.44, almost the same as change-team group.

There were a few more similarities between the groups. In the same-team group, sixteen players moved down, fifteen players moved up and one stayed the same while in the change-team group, four players moved down and five players moved up. Thirteen of the 32 in the same-team group had changes of greater than five runs compared with a close three out of nine for the same-team group. The Cardinals expended a significant amount of money on Peralta to fix their most glaring weakness. The defensive metrics contend Peralta is a solid shortstop, and there is little evidence to suggest changing teams will affect him positively or negatively. Peralta is not a star, but he does not have to be to succeed in St. Louis. If Peralta can provide slightly above average to above average offense with average to slightly above average defense, he will easily prove his worth for the Cardinals as they seek a return to the playoffs.