I'm going to start things off with a graph. Here's how the Cardinals' SS position has fared by year, in terms of fWAR:
Shortstop has been a tough one to fill for the Cardinals. Edgar Renteria had an incredible 2003, ranking second among SS in WAR at a time when Alex Rodriguez, Derek Jeter, Nomar Garciaparra, and Miguel Tejada were all in their primes. It may have been one of the best seasons for shortstops in baseball history, and Renteria was only bested by one of the best players my generation has ever seen.
That was Renteria's last strong year as a Cardinal, as he posted a 2 WAR season the following season, and then ended up signing with the Red Sox, and had a couple strong seasons as a Brave in 2006 and 2007. But for the Cardinals, the shortstop position hasn't been the same since. David Eckstein put up a 2.5 WAR season in 2005 and while everyone remembers the 2006 heroics, he was below average during the regular season, putting up just 1.5 WAR in 552 PA that year. That was the first of eight consecutive years that the Redbird shortstops combined for less than 2 WAR. Enter Jhonny Peralta...
As you know, the Cardinals signed Peralta to a 4 year, $53M deal in the winter of 2013-2014, after the team got less than replacement level value out of a tragic starting duo of Pete Kozma and Daniel Descalso. Jhonny made Cardinals fans forget all about that mess, as he proceeded to lead the team (and all shortstops around the league) in fWAR in 2014. His 120 wRC+, placed him second among shortstops that year to only Hanley Ramirez, who's poor defensive value dragged his value down to two wins less than Peralta.
It was prudent to project regression from there, but it hit a little harder than the Cardinals had hoped. Peralta's value didn't crater, but he was below average on offense and defense, leading to a 1.7 WAR season. Lets look at his recent seasons to see what changed:
|2011||576||6.9 %||16.5 %||.179||.325||.356||122||-4.9||9.5||16.7||4.7|
|2012||585||8.4 %||17.9 %||.145||.275||.301||86||-5.2||-14.8||18||2.4|
|2013||448||7.8 %||21.9 %||.154||.374||.356||124||-0.6||11.8||8.3||3.8|
|2014||628||9.2 %||17.8 %||.180||.292||.343||120||-3.5||10.9||18.8||5.3|
|2015||640||7.8 %||17.3 %||.136||.311||.323||105||-6.6||-3.1||-0.8||1.7|
You can see right away that defense represents a big chunk of Peralta's value. From 2011 to 2014 Peralta was on average a +15 defender, and last year he was rated just below average compared to the rest of the league. While it's possible he lost 16 runs worth of defensive value in a year, and he's probably not going to be as good as 2011-2014 going forward, this is probably more of a matter of inadequate sample size in the defensive metrics, which take three years to be reliable.
As for hitting, Peralta posted his worse ISO since 2009, after a 2014 season that saw his highest ISO since 2008. His walk rate was tied with 2013 for his second lowest walk rate of his career (2011 was his lowest). Of course, even with those things and a BABIP almost right at his career average, he posted a 105 wRC+, which is pretty great for a shortstop (shortstops averaged an 85 wRC+).
But, the time may be closing on Peralta getting the "for a shortstop" caveat to his numbers. He's certainly a poor base runner, especially relative to other shortstops, and if he is losing a step on defense that will bring down his numbers as well. The average shortstop last year was 0.8 runs above average on the base paths over the 640 PA Peralta had in 2015, and the positional adjustment for shortstops is +7.5 runs. So last year UZR graded his defense as 8 runs worse than the average shortstop, and his base running was 7 runs worse. Those 15 runs add up to a win and a half that the average shortstop was better than Jhonny is 2015 in base running and defense.
I think he will rebound as a defender, but its unclear to what extent. What about his numbers at the plate? The drop in walks can be explained by the fact that Peralta posted the highest O-swing% of his career (33.1% when his previous high was 31.8% in 2010, and he was as low as 28.1% in 2013). Pitchers responded, as Peralta also posted the lowest Zone% of his career at 42.3%. To a small but not insignificant degree, Peralta was a little more likely to chase, and pitchers obliged.
The other problem for Jhonny is getting the ball in the air. In 2015 Jhonny posted both his highest GB% and his lowest FB% since 2009. Despite a down year in ISO, his HR/FB beat his career average. Can Peralta rebound? I think so. His numbers have bounced around a lot in his weird career, and O-swing% and GB/FB numbers don't seem like the type of things that just fall off as you age. Peralta may no longer be rated as one of the best infield defenders in the game, but that's to be expected for someone entering their age 34 season. He may have lost a step but players don't typically go from being one of the best to league average over the course of a single year, at least in terms of true talent level.
The projections agree, as Steamer sees Peralta being worth 2.2 WAR this year with Zips more optimistic at 2.6. Last year probably isn't the new normal for Peralta, just as a repeat of 2014's stellar campaign shouldn't have been expected. But the Cardinals are still in a better place now than they have been most of the last decade at shortstop.