A lot has already been said about the Diamondbacks rotation this year, which is currently first in both fWAR and bWAR. But after poor pre-season projections and a rough start for Fernando Rodney, it seemed like the bullpen would be a major concern. However, their performance as a collective whole has thus-far been among the best in the league, depending on how you look at them. So just how good is it?
We’ll start with the most basic measure of pitching performance. The Dbacks bullpen has an ERA of 3.38, which is good for 5th best in the MLB. Note that this is not park-adjusted.
The next easiest way to look at our bullpen is to look at our bullpen’s WAR and compare it to the rest of the league.
FanGraphs: 2.6 fWAR (10th best)
BB-Ref: 4.4 bWAR (4th best)
Baseball Prospectus: 3.97 DRA* (5th best)
*Note that “DRA” stands for “deserved run average”, which is how BP values pitchers. Read more HERE.
Looking at the three main valuation systems, the bullpen ranks between 4th and 10th. Keep in mind that fWAR and bWAR are not rate stats and will therefore be affected by the number of innings pitched. For reference, the Dbacks bullpen has thrown the third fewest innings this season.
By Advanced Pitching Metrics (K%, BB%, K%-BB%, HR/9, FIP)
Our next step is to look at our advanced pitching metrics:
K%: 25.6% (9th highest)
BB%: 9.2% (16th lowest)
K%-BB%: 16.4% (9th highest)
HR/9: 0.97 (8th lowest)
FIP: 3.62 (7th lowest)
Our advanced pitching metrics don’t paint as rosy of a picture as our bWAR/DRA, but we still rank in the top third for most things. The thing that stands out here is that the Dbacks bullpen is pretty consistent across all the main components, which is seen in our 7th-ranked FIP.
By Park-Adjusted Measures (ERA-, FIP-, xFIP-)
So the Diamondbacks sit in 5th per ERA and 7th in FIP, but we also play in one of the most offensive ballparks in the MLB, which is going to naturally inflate things like ERA while also possibly deflating K% and BB%.
ERA-: 74 (4th best)
FIP-: 83 (8th best)
xFIP-: 86 (4th best)
Putting it all together, the park-adjusted metrics continue to view the Dbacks favorably. I’m surprised that our FIP- dropped to 8th (from a 7th-ranked FIP) but that’s really not a big deal.
By Win Probability
This is where things are going to get interesting. So far, we’ve evaluated the performance of our pitching but aside from fWAR, these metrics have so far been considered context-neutral. As we know, part of pitching in the bullpen includes pitching in very high-leverage situations. So how is AZ doing when the game is on the line?
WPA: 4.02 (4th best)
pLI*: 1.13 (6th highest)
gmLI**: 1.29 (2nd highest)
WPA/LI: 2.57 (6th best)
*pLI = A player’s average LI for all game events
**gmLI = A player’s average LI when he enters the game
The Diamondbacks bullpen is performing very well when the game is on the line. Their WPA is 4th-best in the MLB despite pitching such a (relatively) low number of innings. Diamondbacks relievers also enter the game with the second-highest average leverage, which means that when they are called into games, the pressure is on them.
However, win probability and the LI metrics aren’t entirely fair - different players will have different results based upon the situations that they are brought into. To address this, we can look at Context Neutral Wins or WPA/LI. Essentially, it measures the amount of wins added while neutralizing the leverage, so it can be treated as more of a rate stat than WPA (which is strictly a volume stat). WPA/LI is considered to be the best measure of the win-probability talents - and the Diamondbacks still rank 6th in the MLB.
The Diamondbacks bullpen continues to rank very highly - I would say that the win probability metrics favor us more than any other stat. Let’s take a look at one last thing - shutdowns and meltdowns:
Shutdowns: 65 (4th highest)
Meltdowns: 29 (7th lowest)
SD/MD: 2.24 (4th lowest)
Once again, the Diamondbacks bullpen just continues to perform.
So how good is the Dbacks bullpen?
We’ve looked a large number of stats and the Diamondbacks rank very favorably among every stat we’ve looked at - ranking in the top third in everything except BB%. Our advanced pitching metrics aren’t quite as strong, but considering our bWAR, DRA, ERA-, xFIP-, and all of our win probability metrics, I would place the Diamondbacks bullpen as one of the top 4-6 bullpens in the MLB right now. Considering where the bullpen was coming into the season, this might be the second-biggest surprise of the season (behind our starting rotation’s resurgence).
This puts the Dbacks in a very precarious situation going forward - do you try and make an upgrade to the bullpen at the deadline? For a long time, I thought this would be where we could make an easy move but now I am starting to think that an upgrade to the bullpen may be more expensive than it’s worth. But I’ll leave that debate to the comments.