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Who to Acquire for the Diamondbacks Rotation?

Which of 4 free agents would be best?

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Blake Snell after throwing 7 hitless innings.
Blake Snell after throwing 7 hitless innings.
Photo by Sean M. Haffey/Getty Images


One of the biggest things that went wrong in the World Series per Makakilo:

“The Diamondbacks needed an additional pitcher in the rotation. The need would have been for two additional pitchers in the rotation, except Brandon Pfaadt stepped up his game just in time for the playoffs. Reasons for that shortfall:

Madison Bumgarner lost his effectiveness (In April, his last four games averaged just over 4 innings with an ERA of 10.26.

Zack Davies lost his effectiveness (In September, his last 5 games averaged 4 innings with an ERA of 7.20.

Mike Hazen failed to add to the rotation despite many attempts. (“We were buying. I chased every starting pitcher on the free agent market.” — Mike Hazen, November 2023)

Ending September with three starters (Gallen, Kelly, and Pfaadt) meant bullpen games in the playoffs. In the World Series, the Diamondbacks lost that bullpen game.

At the trade deadline, Mike Hazen explored many trade packages (with and without Alek Thomas). Sadly, the trades that were too expensive in terms of prospects. In retrospect he seemed to regret not pushing all-in at the trade deadline for “potential free agent players.” To me that meant he focused on “rental” pitchers who would become free agents in 2024. This season, perhaps he will focus on a multi-year acquisition. For now, I will make that assumption.

Looking forward to next season, the Diamondbacks need to start the season with at least five excellent pitchers in the rotation, so that it is very likely that at least four are available for the playoffs. In the absense of any pleasant surprises in spring training, reaching 5 excellent starting pitchers will require externally acquiring a starting pitcher.

What Factors Could Show the Best Choice?

Let’s look at the following factors to guide our thinking:

  • Fastball Velocity. Although arguably pitching is about fooling the batter with different pitch movements and different pitch velocities, my view is that to avoid batting practice situations, average fastball velocity needs to be at least 94 MPH.
  • Strikeouts per batter faced. Part of successful pitching means the batter often walks away with nothing because he struck out. Success means that at least a quarter of batters strike out.
  • Whiffs. When a batter swings and misses the ball, it is evidence that the pitcher is dominating the batter. Success is defined as at least .13 whiffs per pitch.
  • Balls-In-Play. The batter always has a chance if he puts the ball in play. Good pitchers don’t let that happen very often. Success is defined as fewer than .265 balls-in-play per strike.
  • Consistency adds to a pitcher’s value. Although other measures are possible, I looked at Baseball Reference’s Game Scores to determine consistency. Consistency is defined as less than 10% of Game Scores are less than 40 (a below-average start), and at least 30% of Game scores are more than 60 (an above-average start).
  • When comparing pitchers, Fielding Independent Pitching (FIP), is one way to look at pitchers independent of how good their team defends. Success is defined as FIP of less than 4.
  • Home runs can make a big impact. For the season, home runs per 9 innings should be less than 1.
  • Avoid bums. The Diamondbacks were burned when they signed Madison Bumgarner. When signed, he had pitched 2329 innings (Majors, postseason, and minors). His performance immediately declined dramatically. With the Diamondbacks, he averaged negative 0.1 bWAR per season. To avoid making the same mistake twice, any pitcher within 20% of Bumgarner’s innings pitched (1863 innings) should not be signed to more than a one-year contract.

Four Possible Acquisitions.

Let’s consider four free agent possibilities: Blake Snell LHSP, Jordan Montgomery LHSP, Aaron Nola RHSP, and Sonny Gray RHSP. Which would be best?

Let’s see whether the four possible acquisitions meet our criteria. The following two tables show the strengths and weaknesses of each pitcher.

The first table shows Snell and Nola meet the criteria for strikeouts per batter faced, and balls-in-play. Only Snell met the criteria for fastball velocity and whiffs per pitch.

Data from Baseball Reference and Baseball Savant.

The second table shows two concerns about Nola (his FIP and his home runs per 9 innings).

The second table shows that because of high career innings pitched, the Diamondbacks should only offer 1-year contracts to Nola and Gray. It is unlikely that either would accept a 1-year contract.

Data from Baseball Reference.


Of the four free agent starting pitchers considered, Blake Snell is the only one who meets the criteria. He would be an awesome acquisition. This article speculated he will be signed for 5 seasons and $122 Million. Perhaps the Diamondbacks will be in the mix to sign him.

However, there could be competition from other teams. This article indicated that the Phillies may eschew Aaron Nola and instead attempt to sign Blake Snell.

My conclusion is that due to competition the Diamondbacks may fail to acquire Blake Snell, perhaps the only one of the four really worth acquiring on a multi-season contract.

I am open to more possibilities regarding the Diamondbacks’ rotation. I will continue to look for more candidates.


Blake Snell looks like a good acquisition for the Diamondbacks rotation, but it may not happen. There may be other possibilities.