How good is the Twins' bullpen?

Going into spring training, I was pretty bullish on the Twins' bullpen.  We had one of the best closers in baseball, two right-handed set up men who combined for a 2.25 ERA in 92 innings for the Twins in 2009 (Rauch and Guerrier), two more righties with big question marks but big upsides (Crain, Neshek), and a lefty with a career 2.12 ERA through 81 games (Mijares).  For me, and many Twins fans, there was every reason to expect the team to field a very good, if not great, bullpen in 2010.

Of course, not everything went as planned.  Nathan got hurt, taking away the ace of the pen while shuffling one of the two right-handed set-up men into the role of closer. Neshek only pitched 4 innings before heading to the DL, and is still working his way back to the majors.  Crain has been...well, awful and unreliable.  Those are three big blows to any bullpen, and could have been a major setback to a Twins team poised for great things in 2010.

So, given those setbacks, how is it that the Twins have one of the best bullpens in baseball?  Consider the numbers (after the jump):

Twins 2010 Bullpen, through June 21

• 2.89 ERA, second-lowest in the AL
• 1.15 WHIP, lowest in the AL
• 2.42 walks per nine, lowest in the majors
• Three of the top-15 (and four of the top-25) relievers in the AL, based on Expected Wins Added (Guerrier, Rauch, Duensing, and Burnett, in that order)

Some more specifics on the key contributors, ordered by innings pitched:

It's easy to see why this bullpen has been so successful: the four guys pitching the most innings out of the bullpen have been filthy, with ERAs ranging from 1.69 to 2.70.  They throw strikes, limit walks, and keep the ball in the park. 

But are they really as good as their ERAs suggest? 

For those familiar with defense-independent pitching statistics like FIP and xFIP, some of these numbers should give you pause (for those who aren't familiar, read here and here).  The low ERAs sported by the workhorses of the bullpen - Burnett, Guerrier, and Duensing - are backed by less than stellar peripheral numbers, and perhaps a bit of luck:

• Burnett and Guerrier are both sporting unsustainably low home run per fly ball rates.  On average, about 10% of fly balls hit off a pitcher will reach the seats.  Anything significantly lower than that (like, say, 3%) could be a sign a pitcher is getting lucky.  Note: These numbers don't reflect Burnett's poor outing Tuesday night.
• Both Guerrier and Duensing have seen an unexpectedly high number of balls in play find fielders' gloves.  Duensing's BABIP is currently 7^th lowest among qualified major league relievers.  Guerrier comes in at 15^th.
• In total, the Twins currently sport the highest differential between their bullpen ERA (2.89) and FIP (3.82), and the second highest difference between their ERA and xFIP (4.28). 

By no means am I trying to suggest the Twins' bullpen is teetering on the brink of collapse.  Even accounting for luck, the Twins' bullpen still ranks 4^th in the AL in FIP and 5^th in xFIP.  Rauch is a very good reliever, even if his ERA is a little lower than we should expect.  Guerrier just plain confounds FIP-heads every year (seriously, check it out).  Alex Burnett was extremely good at limiting homeruns in the minors, too.  Left-handed hitters just can't seem to hit Brian Duensing, meaning that if he's deployed strategically his overall numbers could understate his value.  Crain's peripherals are encouraging, if not overwhelming.  Neshek could return at any time, and has the potential to regain his status as an elite right-handed reliever.  On top this, Anthony Slama and Kyle Waldrop are pitching extremely well in AAA, and could become key pieces of the bullpen before the season's end.

The bottom line, in my view, is that this is a good bullpen that has pitched like a great one so far in 2010.  We've had solid contributions from guys who were fighting for spots on the roster in spring training, and multiple pitchers waiting in the wings that look ready to contribute at the big league level.  Yes, we're seeing the pen pitch above their heads right now, and we should expect to see their performance come back to Earth soon.  But given the concerns we had earlier this spring, the Twins' bullpen continues to be a pleasant surprise.