A Perspective of the Diamondbacks’ Rebuilt Bullpen

If the season started this week, let’s look at my choices for bullpen pitchers, and what characteristics would define that bullpen.

Previously, I looked at Which Relief Pitcher is Worth Acquiring via Trade?

Two foundational requirements were average fastball velocity of at least 94 MPH, and age 30 or younger. Of the pitchers I considered, I found that David Bednar had the highest strikes per pitch and the lowest balls in play per pitch. Also, his whiffs (swinging strikes without contact) were generally higher than his called strikes.

My view evolved. Looking at the rebuilt Diamondbacks bullpen, important characteristics are:

• age
• fastball velocity.
• strikeouts per batter faced.
• whiffs per pitch.
• balls in play including homers per strike.
• ZiPS projected FIP.

When ZiPS projected a pitcher to have more games as a starter than as a reliever, I excluded them from my promising candidates. One such exclusion was Corbin Martin, who has been discussed as a bullpen candidate.

Although the pitchers listed in my tables were most promising in my view, I may have omitted promising candidates. Feel free to advocate for other pitchers in the comment section.

Fastball velocity will be important. In 2017, the Diamondbacks’ 92.8 MPH average fastball velocity was the eighth fastest in the Majors per Baseball Savant. Since then (with the exception of 2020) the Diamondbacks’ fastballs were one of the slowest three teams. Last season’s 91.7 MPH was the slowest in the Majors.

Next season will likely be different. Of seven recently acquired bullpen pitchers, four had average fastball velocities greater than 94 MPH.

Pitchers sometimes have more than one type of fastball. In player tables, if their fastest type of fastball had an average speed of at least 94 MPH, the number is green.

Strikeouts per batter faced will be important. Perhaps, this statistic measures having great put-away pitches. My criteria is one strikeout for every four batters faced. In player tables, when that level is reached the number is green.

Whiffs per pitch will be important. With next season’s limitations on defensive shifting, pitching to contact may be less effective. Whiffs become more important. In my player tables, when whiffs per pitch are at least .130 the number is green.

Avoiding balls in play (including homers) will be important. In 2022, the Diamondbacks’ pitchers allowed the fifth most home runs in the Majors. Although the Diamondbacks relief pitchers performed better, .029 vs .033 homers per batter faced (HR/BF), the relief pitcher homers per batter faced ranked the eighth highest in the Majors.

Although the Diamondbacks’ defense is well above average in the Majors, excessive balls in play can hurt the team. Last season, Diamondbacks’ pitchers in a relief role had .704 balls in play per PA per Baseball Savant, which was the highest in the Majors.

In player tables, when balls in play (including homers) per strike is less than or equal to .265 the number is green.

As always, Fielding Independent Pitching (FIP), which measures a pitcher’s effectiveness, is important. In player tables, when FIP is less than 4 the number is green.

Who would be my starting bullpen if the season started on 20 December?

Five pitchers met at least two of the three criteria (strikeouts per batter faced, whiffs per pitch, and balls in play including homers per strike). Immediately chosen were Castro, Hendrix, McGough, Sulser, and Mantiply.

Kevin Ginkel was chosen because of his 96.4 average fastball velocity, because he met the balls in play criteria, and because his statistics were not to far away from meeting the other two criteria.

Mark Melancon was chosen because he could bounce back and surprise fans. Moving on quickly to another difficult choice.

Three pitchers were considered for the last bullpen position. The following table compares them on the three criteria. Kyle Nelson was the best of the three pitchers for two of the three criteria.

Data from 2022 season: Baseball Reference,

Now you know how the pitchers included in the following table were chosen.

Diamondbacks’ bullpen on 20 December, 2022. Data from 2022 season: Baseball Reference, Baseball Savant, and FanGraphs.

Who would be my depth bullpen pitchers in the minors?

I have high hopes that the pitchers in the following table will continue to develop and perhaps will be called up during the season.

Luis Frias’s fastball velocity would be the second highest in the bullpen. His 3.99 projected FIP is excellent.

Tyler Holton’s 1.51 whiffs per pitch met one criteria. His 3.84 projected FIP is excellent.

If Austin Brice could bounce back to how he pitched in 2020, he would meet all three criteria.

Justin Martinez and Carlos Vargas have high velocity fastballs and in AAA their strikeouts per batter faced are impressive. They are exciting prospects.

Diamondbacks’ bullpen depth in minors on 20 December, 2022. Data from 2021 & 2022 seasons: Baseball Reference, Baseball Savant, and FanGraphs.

Which pitchers meet my criteria for strikeouts, whiffs, and avoiding balls in play?

Six pitchers near the center of the following diagram best meet the three criteria. My bullpen would include all of them. The other chosen bullpen pitchers (not on the Diagram) were Mark Melancon and Kyle Nelson.

Diamondbacks’ bullpen pitcher characteristics on 20 December, 2022. Data from 2022 season: Baseball Reference and Baseball Savant.

How would choosing by projected FIP be different?

First, I tip my hat to Heath Klein who commented about going with Diamondbacks relievers with the best projected FIPs. How would a projected FIP perspective differ from my criteria? If bullpen choices were made based on projected FIP instead my criteria, and assuming that McGough is in without a projected FIP, the following would be different:

• Tyler Holton would join the bullpen and Ryan Hendrix would become bullpen depth.
• Luis Frias would join the bullpen and Kyle Nelson would become bullpen depth. Frias’s average fastball velocity would be the second highest in the bullpen.

Summary.

In each of the past 5 seasons, except for 2020, the Diamondbacks’ average fastball velocities ranked in the slowest 3 teams. That will likely change next season because they acquired four bullpen pitchers with velocities greater than 94 MPH.

My view of the characteristics of effective bullpen pitchers, and the criteria for each characteristic, evolved into the following:

• One strikeout for every four batters faced.
• Whiffs per pitch of at least .130.
• Balls in play (including homers) per strike is less than or equal to .265.

Those criteria led to six clear choices for the Diamondbacks’ bullpen as follows:

• Miguel Castro.
• Kevin Ginkel.
• Ryan Hendrix.
• Scott McGough.
• Cole Sulser.
• Joe Mantiply.

The choices for the last two bullpen positions follow:

• Mark Melancon.
• Kyle Nelson.