Pressure Makes Diamond(back)s: Arizona Bullpen Clicking at the Right Time

Kiyoshi Mio-USA TODAY Sports

The 2023 regular season was a tumultuous one for the Arizona Diamondbacks. They were one of baseball’s best teams in the first half, with the fourth-best record in the league on July 1. Then in the month of July, they went just 8-16, tied for the second-worst record in baseball. They started August off on a nine-game losing streak before going 11-2 in their next 13 games. The rest of the season would be much of the same – losing five of six, then winning five of six, and so on and so forth. The club secured a playoff spot with an 8-2 stretch from September 15–27, before losing its final four regular-season games. From a playoff-odds perspective, the roller coaster looked like this:

If one of the defining characteristics of the Diamondbacks’ 2023 regular season was volatility, the bullpen was no exception. After a slow start, the group had a strong May, collapsed to be one of the league’s worst in July, and battled back to have a quite effective final month. By the time they’d played 162, 30 pitchers had appeared in relief for Arizona – just two NL teams used more. Entering a short-series playoff format (have you heard about the short-series playoff format?), the pressure was on for the rostered subset to find their best, holding leads when they had them and keeping games close when they didn’t. Through two rounds, they’ve more than answered the call:

Diamondbacks Bullpen Ranks
Month ERA FIP xFIP
March/April 22 21 22
May 8 8 6
June 20 10 18
July 28 29 27
August 20 19 12
Sept./Oct. 8 6 12
Postseason* 3 2 2
*Among 12 teams

In five games so far this postseason, Arizona’s bullpen has carried one of the heaviest loads, eating up 20.1 innings and allowing just four earned runs. In the Wild Card sweep over the Brewers, the bullpen threw 9.1 scoreless innings, never with a lead of more than three runs, including five frames with a one-run lead in Game 1. The offense made the Division Series a little easier on them – the Dodgers never led – but still, in 11 innings of work, Diamondbacks relievers allowed just a pair of runs in the eighth inning of an 11-run Game 1 and another pair on a string of seventh-inning singles in Game 3. Through the tournament so far, they’ve kept the ball on the ground to the tune of a 57.1% groundball rate and just a 20.4% fly ball rate, without allowing a single home run. They’ve had the highest average leverage of any bullpen, and delivered the most win probability added in those situations – even when adjusted for leverage:

Postseason Bullpens by Win Probability Metrics
Team WPA pLI WPA/LI
ARI 1.23 1.47 0.51
TEX 0.91 0.99 0.45
PHI 0.46 1.29 0.45
HOU 0.58 0.75 0.39
LAD 0.2 0.3 0.34
MIL 0.13 1.1 0.27
TOR 0.03 0.58 0.24
MIN 0.58 0.67 0.24
BAL -0.02 0.33 0.23
TBR -0.06 0.14 0.11
MIA -0.07 0.29 0.01
ATL 0.14 0.53 -0.25

Of the 13 pitchers on the Diamondbacks’ Opening Day roster, just five were on the club’s active Wild Card Series roster. Zac Gallen and Merrill Kelly, the horses upon which they’ve built their starting rotation, were present on both rosters, as was 25-year-old rookie Ryne Nelson, who made 27 starts and a pair of late-season relief appearances for Arizona during the regular season, and was available in relief. That trio handled the bulk – nearly 63% – of the Diamondbacks starters’ innings over the course of the 162-game schedule. But when it came to the regular season bullpen, just two pitchers – Kevin Ginkel and Miguel Castro – made it from the visiting clubhouse in Los Angeles on March 30 to the one in Milwaukee on October 3. Kyle Nelson, who spent the season in Arizona’s bullpen and finished second only to Castro in appearances, was left off the Wild Card roster in favor of righty Bryce Jarvis, though he would return to Dodger Stadium as part of the Division Series roster.

The rest of the playoff ‘pen was pieced together over the course of that six months’ time. Among their internal options, left-hander Joe Mantiply battled shoulder and hamstring injuries (the former costing him his spot on the Opening Day roster), and endured some back-and-forth between Arizona and Triple-A Reno before sticking with the big league club in August and September. Right-hander Luis Frías also shuttled between Reno and the majors, finishing strong in the last of his four stints with the Diamondbacks in September. Young lefty Andrew Saalfrank broke through the upper levels of the minors to earn his major league debut in September, impressing enough in 10 outings without an earned run to earn a spot.

With the postseason in sight, the Diamondbacks also bolstered their relief corps with a few late-season acquisitions. At the trade deadline, they scored closer Paul Sewald from the Mariners. Sewald gave Arizona an established late-inning stalwart – after blowing his first save opportunity, he would convert 13 of his last 14 heading into the playoffs. In late August, more help came in the form of soft-throwing side-armer Ryan Thompson, who had been released by the Rays earlier that month, and whose late summer stint with Arizona included just eight baserunners – and one earned run – in 13 innings:

Diamondbacks WC/DS Relievers in September
Name IP ERA FIP xFIP
Luis Frías 13.2 1.98 4.06 5.17
Ryan Thompson 12.0 0.75 3.51 3.25
Joe Mantiply 11.2 0.77 3.17 4.87
Miguel Castro 11.0 0.00 1.16 1.77
Kevin Ginkel 11.0 3.27 1.98 3.94
Andrew Saalfrank 10.1 0.00 3.26 3.74
Bryce Jarvis 9.0 1.00 3.03 5.05
Paul Sewald 8.0 2.25 4.01 4.24
Kyle Nelson 7.0 9.00 7.83 5.33
Ryne Nelson 5.2 0.00 2.55 5.18

Thompson – the newest member of the group – has been one of the keys to Torey Lovullo’s bullpen management so far in the postseason. He got 12 outs over three holds in Games 1 and 2 of the Wild Card Series and Game 2 of the NLDS before allowing a pair of runs in Game 3. He’s racked up more individual WPA than any other postseason pitcher thus far. Thompson, who has World Series experience with the Rays, has been a great tool for Lovullo as a righty with career reverse splits. With Arizona’s right-handed rotation, the Diamondbacks have been facing lefty-heavy lineups, and Thompson has made for a good option against the toughest-to-manage-against sections of those lineups. Instead of attacking Freddie Freeman with a left-hander who would be faced with Mookie Betts before and Will Smith or J.D. Martinez after, Thompson gives Lovullo a more comfortable matchup against Freeman without the vulnerability of a lefty pitching to lefty-mashers in Betts and Martinez. Thompson handled that top of the order in each of his Division Series outings – he only got in trouble later on in Game 3. In one of his Wild Card appearances, Lovullo used him similarly against the number nine hitter, righty Tyrone Taylor, into the top of the Brewers order, which went lefty Christian Yelich to righty William Contreras to switch-hitter Carlos Santana:

Ryan Thompson’s Career Splits
Handedness TBF AVG OBP SLG wOBA
vs. L 163 .212 .286 .315 .262
vs. R 387 .232 .289 .388 .294

In the upcoming NLCS, Lovullo will look to deploy this bullpen against a dangerous Phillies lineup, and he could continue to use Thompson in big spots against a top three that features left-handed Kyle Schwarber, right-handed Trea Turner, and left-handed Bryce Harper. That still leaves him with lefties Mantiply, Saalfrank and Kyle Nelson to use for different looks at different times in the series.

Ginkel and Sewald have also been used in each game except the NLDS Game 1 blowout, locking down the late innings, and neither has allowed a run. Ginkel was a strong late-inning option for much of the Diamondbacks’ season, and he, too, has been effective against hitters on both sides this year. Through four outings, he has had little issue, fanning eight of his 17 batters faced (47.1%), including six of eight left-handed hitters faced, and five of those on elevated fastballs. Sewald has had a couple of near-misses in terms of serious trouble – a hard-hit Yelich double off the wall in the Wild Card Series and a would-be game-tying home run off the bat of Chris Taylor that found Alek Thomas’ glove on the warning track in the NLDS clincher. He’s never been a groundball pitcher, so when he does allow contact, there will be some held breath in Arizona. But he’s managed to avoid the trouble altogether thus far, and the results have been four zeroes and four straight saves. If he can continue to get opposing hitters to miss on the 45.2% of swings they have so far, he can hope to end up with enough strikeouts to limit any fly ball damage:

Diamondbacks Relievers’ Postseason GB%
Name IP GB%
Luis Frías 1.0 100.0%
Andrew Saalfrank 1.1 100.0%
Ryan Thompson 5.0 71.4%
Joe Mantiply 2.2 66.7%
Kevin Ginkel 4.1 66.7%
Miguel Castro 1.2 60.0%
Ryne Nelson 0.1 33.3%
Paul Sewald 4.0 0.0%

The rest of the Arizona bullpen has bridged the rotation to the late-inning trio of Thompson, Ginkel, and Sewald more than capably. Mantiply has served as a great heir of inherited baserunners, stranding all four he’s taken on and retiring eight of nine batters faced in three outings. Saalfrank, too, has been used as a sort of tourniquet of messy innings, entering to runners on in each of his three outings and only letting one come in. Frías made a spotless appearance in garbage time of the blowout. Castro and Ryne Nelson have struggled in limited opportunities, but the rest of the cast has been strong enough to cover them.

So far – in only five games, mind you – it’s worked like a well-oiled machine. By keeping the ball mostly on the ground, with a strong defense backing them up, they’ve managed to keep offenses from getting too much going. Even in the cases where teams have been able to string some hits together, they’ve mostly been groundball singles, giving Arizona a chance to limit the damage.

The NLCS, though, will bring a different level of adversity to this group. The Dimondbacks played their first five games of the postseason over nine days, with little strain on their pitching staff from a usage standpoint. In the Division Series, the staff was given a multi-run lead before the starter even took the mound in Games 1 and 2, and a four-run lead in the third inning of Game 3. The bullpen still had to perform against a supremely talented Dodger lineup, but they’ve hardly had their backs against the wall since Game 1 of the Wild Card Series. A best-of-seven series is a whole different animal – especially when it’s only Gallen and Kelly that Arizona can trust to give them any real length from the starting rotation. There’s some vulnerability in the middle innings, and it’s harder to get through a best-of-seven without that vulnerability getting exploited.

One matchup-specific red flag for Arizona: The Phillies tend to feast on low-velocity fastballs. Their .513 slugging percentage on fastballs thrown 94 mph or slower ranks fourth in the majors. And during the regular season, 68.7% of Arizona’s fastballs came in at or under that mark – that’s the third-most in baseball. Among the Diamondbacks’ postseason staff, Gallen, Brandon Pfaadt, Saalfrank, Sewald, Kelly, Kyle Nelson, Thompson, and Mantiply are all in the 40th percentile in fastball velocity or lower. As a team, they’ve also thrown more fastballs this postseason than any other:

Arizona’s FB Velocity Percentiles
Player Percentile
Miguel Castro 90
Luis Frías 85
Kevin Ginkel 76
Bryce Jarvis 73
Ryne Nelson 56
Zac Gallen 40
Brandon Pfaadt 40
Andrew Saalfrank 24
Paul Sewald 22
Merrill Kelly 22
Kyle Nelson 19
Tommy Henry 10
Ryan Thompson 8
Joe Mantiply 6
SOURCE: Baseball Savant

The Arizona bullpen has been a pleasant surprise through the first two rounds of the playoffs – then again, so has the rest of the team. Enough has been made of what short-series upsets mean, but the bottom line is, as much as we analyze what has or hasn’t worked through two October rounds, which of these teams comes out of the NLCS will be result of a whole host of factors, and not least of all luck. For us die-hards, that can be either liberating or petrifying, but if it weren’t the case, there’d be no sense in playing the games. The Diamondbacks have a hard road ahead, but if they’ve made it this far, what’s stopping anther run of timely performance from carrying them for eight more wins?





Chris is a data journalist and FanGraphs contributor. Prior to his career in journalism, he worked in baseball media relations for the Chicago Cubs and Boston Red Sox.

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Ivan_Grushenkomember
7 months ago

The Postseason Phillies roster is better than the Postseason Brewers or Dodgers roster so this seems like a tall order for the pen. They would need Wheeler and Nola to go 7+ and the offense to score a lot in the other games.

Ashburn Alley
7 months ago
Reply to  Ivan_Grushenko

Huh? Did you mean Gallen and Kelly rather than Wheeler and Nola?

Ivan_Grushenkomember
7 months ago
Reply to  Ashburn Alley

Yes I meant that.