The Arizona Diamondbacks improved from 65 wins to 94 wins in 2011, an improvement almost entirely driven by pitching and defense, as they scored only 18 more runs than in 2010. A certain percentage of this improvement was credited to the outstanding defense of left fielder Gerardo Parra, center fielder Chris Young and right fielder Justin Upton. Parra won a Gold Glove and the other two had good cases.
When you look at three different fielding metrics, however, Arizona's collective outfield defense wasn't any better than it was in 2010:
Defensive Runs Saved, outfield:
2011: +21 runs
2010: +38 runs
2011: +31.1 runs
2010: +30.1 runs
2011: +14 runs
2010: +47 runs
In fact, two of the three defensive metrics suggest Arizona's outfield defense, while still strong, wasn't as good as 2010, when the club lost 97 games.
So, while my first response to the Diamondbacks signing Jason Kubel to a two-year, $15 million contract was, "Why break up this outfield?", my second response was, "You know, maybe they could use another bat in the lineup instead of Gerardo Parra." Arizona finished fourth in the National League with its 731 runs in 2011, just one game from finishing second (735). However, keep in mind they play in a hitter-friendly park.
Anyway, we can evaluate this question by asking: Will Kubel create more runs on offense compared to Parra than he'll cost on defense compared to Parra?
Parra on defense (DRS, UZR, Total Zone):
2010 (849 innings): +20, +14, +16
2011 (1,101 innings): +12, +10, +3
It's interesting to note that all three metrics rated Parra's defense lower in 2011, even though he played more innings. He's still an excellent left fielder, but his metrics were off-the-charts good in 2010.
Here are Kubel's defensive runs per 1,100 innings over the past two seasons (DRS, UZR, Total Zone):
2010: +1, -16, +1
2011: -7, -6, +9
Well ... not a lot of consistency in those ratings. His reputation is that he's not a good outfielder, confirmed by the fact that the Twins started him at DH in about a third of his starts the past two seasons. Taking the best and worst totals from 2011, Kubel is a -7 defender and Parra a +12, a difference of 19 runs over 1,100 innings.
Does Kubel make that up with his bat? Not in 2011, when he hit .273/.332/.434 while Parra hit .292/.357/.427. Of course, you have to account for the fact that Parra played in a good hitter's park, drew 16 intentional thanks to batting eighth and hadn't been a good offensive player prior to 2011. Kubel also doesn't hit lefties well, essentially making him a platoon player.
Over the past two seasons, using wRC (weighted runs created via Fangraphs.com), Kubel has created 119 runs in 982 plate appearances, Parra 91 runs in 886 plate appearances. Over 500 plate appearances, that's an advantage of about 10 runs for Kubel.
I can understand the Diamondbacks' desire to add a left-handed bat with some power: They have Upton, Young and Paul Goldschmidt from the right side, but only Miguel Montero from the left side (plus Stephen Drew, if he's healthy and if he hits). But when you do the math, it's difficult to come up with a scenario where taking away playing time from Parra for Kubel makes the Diamondbacks better.
Now there are three final notes to add:
1. Maybe Kubel will hit like he did in 2009, when he hit .300/.369/.539.
2. Maybe Arizona thinks Parra's 2011 season was a fluke and he'll regress back to previous levels. But he was only 24, and his second half was just as strong as his first half. It would seem his growth at the plate was real.
3. It does give Arizona some much-needed depth. They get didn't any production from reserves like Xavier Nady and Collin Cowgill. Kubel hasn't played first base in his career, but maybe he can spell Goldschmidt against some tough right-handers. Parra can fill in at all three outfield spots, replace Kubel late in games on defense or provide a solid pinch-hitter off the bench. I can still see him getting 300 plate appearances.
In the end, the depth is good, but it doesn't come cheap. I don't think this signing makes Arizona better, but it does give them that extra insurance policy -- a $15 million insurance policy. Either that or maybe a trade is in the works. After all, I can't remember the last time an elite defender like Parra lost his job in the offseason.