What's Going On With: Joe Nathan

Year	Age	Tm	Lg	W	L	ERA	G	GF	SV	IP	H	ER	HR	BB	SO	ERA+	WHIP
2012 ★	37	TEX	AL	3	5	2.80	66	62	37	64.1	55	20	7	13	78	157	1.057
2013 ★	38	TEX	AL	6	2	1.39	67	61	43	64.2	36	10	2	22	73	297	0.897
2014	39	DET	AL	3	2	6.57	27	22	13	24.2	27	18	4	13	21	64	1.622

Provided by Baseball-Reference.com: View Original Table
Generated 6/18/2014.

What's The Story? The Tigers, perennially in need of bullpen help, signed closer Joe Nathan as a free agent over the winter. Some thought that the team's other two big trades, sending first baseman Prince Fielder to Texas and starter Doug Fister to Washington, were made in part to clear salary space for Nathan, whom the Tigers are paying $9 million this year and $10 million in 2015. Though this article wondered whether age might catch up with Nathan, his track record through age 38 was pretty unimpeachable. Here's a list of saves leaders between the ages of 29 and 38, a period during which Nathan lost a year and a half due to injury:

Player	SV	G	GF	W	L	IP	H	ER	BB	SO	ERA	ERA+	HR
Mariano Rivera	398	651	598	46	39	716.0	543	166	143	649	2.09	217	33
Trevor Hoffman	384	582	502	25	36	598.0	458	167	146	655	2.51	159	50
Joe Nathan	340	593	517	33	20	592.1	400	141	169	712	2.14	204	43
Lee Smith	329	601	501	35	50	670.0	598	228	246	688	3.06	134	53
Billy Wagner	315	601	510	28	22	622.1	415	147	174	774	2.13	205	54
Francisco Cordero	304	609	495	37	40	598.0	539	220	256	596	3.31	133	45

Provided by Baseball-Reference.com: View Play Index Tool Used
Generated 6/18/2014.

It hasn't worked out as planned. As you can see from the table at the top, Nathan's been terrible. Among American League relievers with at least 20 innings pitched, he ranks 80 out of 83 in ERA and 75 in WHIP. He's a big contributor to Detroit's league-worst 4.67 relief ERA. He's striking out fewer batters and walking more than any time since he became a reliever in 2003.

If You Don't Want to Read All the Rest of This: He's not getting batters to chase pitches outside the strike zone, he's shown a propensity for the longball, and, well, he's old. 

What's He Throwing?
Against Lefties:
Year   Fastball Slider  Curve  Sinker   Change
2012      48%     19%     22%     12%      0%
2013      28%     33%     14%     26%      0%
2014      45%     18%     22%     18%      3%

Against Righties:
Year   Fastball Slider  Curve  Sinker   Change
2012      53%     35%      7%      5%      0%
2013      50%     39%      5%      5%      0%
2014      40%     50%      5%      4%      0%

Overall:
Year   Fastball Slider  Curve  Sinker   Change
2012      50%     25%     16%      9%      0%
2013      38%     35%     10%     16%      0%
2014      45%     33%     14%      9%      2%

There's really not a lot of change in his repertoire overall, but he's changed his approach toward both lefties (more fastballs, fewer sliders--much like he pitched in 2012) and righties (fewer fastballs, more sliders). Lefties are pounding him (.391 on base percentage, .455 slugging percentage - that's basically Tony Gwynn's career) and righties are hitting him harder (.320 on base, .395 slugging) than they have in years.

What Are The Results?
Year      Zone%    O-Swing%   HR/FB%

2012      51.1%     32.4%     13.0%
2013      49.4%     31.9%      3.0%
2014      48.9%     27.6%     13.8%
Career    48.1%     33.1%      7.3%

Remember the rule: It's good to throw strikes, but better to get strikes. So far this year, 49% of Nathan's pitches are in the strike zone (Zone%). That's down a little from recent years, though it's still OK. More worryingly, though, he's getting batters to swing at less than 28% of pitches outside the strike zone (O-Swing%). Among AL relievers with 20 or more innings, he's middle of the pack (41 of 83) at pitching in the strike zone but 54th at getting batters to chase. Last year he was in the top third at inducing swings outside the strike zone.

Nathan likes to pitch to the left side of the plate (inside to right-handed hitters, outside to lefties). In 2012, he got batters to swing at 31% of pitches outside the strike zone thrown to that side of the plate. It was 29% last year but plunged to 24% this year. As a result, he's walking more batters. Plus, when batters hit balls in the air, they're going out more, as his ratio of home runs to fly balls (HR/FB%) is way up, from 3% last year to 14% this year.

This is not about a lack of control, as his Zone% indicates. It's about his pitches outside the strike zone being easier to lay off, and those in the strike zone easier to hit.

What Does the Future Hold? His home runs per fly ball percentage has been wildly eccentric the past few years and could very well stabilize at closer to his lifetime average of 7% or the league average of 11%. That'd help lower his home run rate.

But his inability to get strikes on pitches outside the zone...that's a problem. Batters are laying off those pitches and waiting for him to throw in the zone, where they're batting .377 against him. There's not an obvious problem, like a lack of velocity or an inability to throw strikes. Maybe it's just that time's catching up with a 39-year-old arm. But unless he can start fooling hitters again, the ninth inning could be tough for yet another year in Detroit.

(All data here from Brooks Baseball and Baseball-Reference.)