UPDATE: Joe Nathan allowed two hits, two walks and four earned runs in one-third of an inning in last night’s loss to the Toronto Blue Jays.

The Detroit Tigers are in firm command of the American League Central Division, posting a 31-22 record that gives them a five-game lead on their closest rival, the Chicago White Sox. Their Achilles’ heel, however, has been their bullpen.

Detroit’s relief pitchers rank 25th in the league with an ERA of 4.29 but are tied for ninth with a save rate of 70 percent, thanks in large part to Joe Nathan’s performance in the closer’s role.

Nathan had just three blown saves in 46 opportunities last year. This year, he has blown four chances in 17 opportunities, putting him on pace for 12, which would be the most since Ambiorix Burgos’s 2006 campaign for the Kansas City Royals. But the Tigers shouldn’t let it get that far because there are a bunch of warning signs.

Nathan’s fastball and slider have lost their zip

Since velocity has been tracked during the 2007 season, Nathan’s fastball has been clocked at a high of 95.1 mph. Now it  sits at a medicore 91.5 mph, a huge decline for a pitch he throws a majority of the time (42.5 percent). His other bread-and-butter pitch, the slider, has also seen a drop in velocity (88.8 mph in 2007 to 85.8 mph this season, according to Pitchf/x).

Nathan’s command is deteriorating

As you would expect, decreased velocity comes with a lower strikeout rate plus a worsening strikeout-to-walk ratio. This season, Nathan is striking out just 2.2 batters for each one he walks, almost a third of his 2012 rate with the Texas Rangers.

Season Team IP SV BS Pulls K/9 BB/9 K/BB K%
2012 Rangers 64.1 37 3 4 10.9 1.8 6.0 30.4%
2013 Rangers 64.2 43 3 6 10.2 3.1 3.3 29.2%
2014 Tigers 20.2 13 4 3 8.7 3.9 2.2 23.0%
Source: Fangraphs

Nathan is not performing in clutch situations

In the 11 games this season where there was a save opportunity with a leverage index of 1.5 or higher, Nathan has seven saves in 9.2 innings with an ERA of 10.24 — the highest among any reliever seeing at least 10 of these opportunities — plus 11 strikeouts and six walks. Last season, he had 22 saves in 25 of these “pressure-packed” situations with an ERA of 3.00,  8.25 strikeouts per nine innings and a 2-to-1 strikeout to walk ratio.

It could be argued that Nathan’s issues are tied directly to the long ball (nine of the 12 runs allowed have come on home runs) but his  xFIP, which measures what a player’s ERA should look like after replacing a pitcher’s home run total with an estimate of how many home runs they should have allowed, would be 4.01, eighth-worst in the league among relievers with at least five saves.

There is another option available to Tigers Manager Brad Ausmus: Joba Chamberlain.

Chamberlain has struck out 29 of the 96 batters faced this season (11.2 strikeouts per nine innings plus  a 4.14 strikeout-to-walk ratio) and has yet to yield a home run, likely a by-product of an increased groundball rate and lower percentage of flyballs on balls put in play.

It’s clear that the changing of the guard in Detroit is overdue and Chamberlain deserves the shot at the closer’s role. Perhaps permanently.