How Good is the Tigers Bullpen?

(Note: This is a guest post, offered up by frequent commenter Chris Hannum. Chris was kind enough to put this together and I felt it should be seen by all the Tigersphere.)

By Chris Hannum

The story of the Tigers so far this year, if the mainstream press is any guide, is that of a team with a struggling patchwork rotation which has been bailed out time and again by an exceptional bullpen.

The question has become: if the Tigers’  starters can’t pitch deeper into games, will the bullpen simply break down from overuse?

If we look at traditional metrics like ERA and winning percentage the Tigers starting rotation has indeed been abysmal.  The ERA for Tigers starters is last in the American League, at 5.76 compared to the next worst mark of 5.15 for Kansas City.  Tigers’ starters have a combined record of 8-10 despite decent offensive support.

The Tigers bullpen, on the other hand, has far and away the best ERA in the AL at 2.17 compared to the next best mark of 2.86 by the Twins.  The bullpen has put up a combined record of 9-4, and would appear by that measure to be the sole reason the Tigers remain above .500.

The Tigers’ bullpen has been pitching more than most, bullpen innings per game is second in the league behind Texas, but they are only averaging 1/3 of an inning per game more than the league as a whole.  Part of that can also be attributed to the 19 road games to only 12 home games they have had so far this year.

The big question shouldn’t be whether or not the Tigers’ pen will crack under the strain, but whether they can really keep this up.

The folks at ESPN give us a few tools we can use to look at whether a staff has been great, or just lucky.  The first of these is what they call the Defense-Independent ERA ratio (DIP%).

A prevailing theory in the sabermetric community is that the control a pitcher has over whether or not a ball in play falls for a hit is minimal at best.  It’s true that line drive rates and the like do seem to matter, but individual pitchers have very little consistency there from year to year.

If we look at the Balls-In-Play-Average (BIPA or BABIP) for a pitcher and see that it is significantly lower than the league average, like for the ’08 incarnation of Armando Gallaraga we would probably guess that what is at work is some combination of great defense bailing him out and pure luck.

The DIP% for the Tigers bullpen is a sky-high 168% (no other team in the AL is higher than 128%) meaning that with an average defense and average ‘luck’ we would expect the Tigers bullpen to have allowed quite a few more hits and have an ERA around 3.65 which would still be good enough for 5^th in the AL.

Here’s what the numbers look like for the Tigers so far: (after the jump)

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So… is the Tigers defense great, or are Tigers relievers getting more than their share of lucky breaks?

If the Tigers defense was getting to a lot of balls, we would expect to see the same skewed numbers from the rotation, with low BIPA numbers and a high DIP%.  In reality, we see the opposite.  BIPA is unusually high for the Tigers rotation & DIP% is 72%, so with average defense and luck the rotation could expect an ERA of 4.11 instead of the ghastly 5.76 we have seen.  This would be good enough for fourth best in the American League as opposed to dead last.

And, of course, with fewer balls falling for hits pitch counts would drop and we could stop worrying about whether or not Joel Zumaya’s arm will fall off.  The key here is that with roughly the same defense backing starters and relievers it doesn’t seem likely that the reason for the ultra-low BIPA numbers for Tigers relievers is the stellar play of Jackson, Inge and Everett.

Our best guess would have to be that lady luck has favored the likes of Eddie Bonine this spring, but likely won’t as the summer wears on.

As a consolation, it looks like the Tigers’ bullpen is still pretty good luck aside and if the problems Scherzer and Porcello have been having are indeed mostly bad luck the rotation should look much better down the stretch than it has thus far.