How Will the Diamondbacks Rotation Fare in 2021?
The Diamondbacks had playoff aspirations entering this past season after acquiring star outfielder Starling Marte and signing Madison Bumgarner to a five-year, $85 million deal. Their win-now deals did not work out, with Marte finishing the year in a Marlins uniform and the team slumping to a 25-35 record that was good for dead last in the NL West. Their team ERA of 4.84, placing them 20th in the majors and second-to-last in the division, was a big contributor to their disappointing season. The Diamondbacks rotation was supposed to be one of their biggest strengths, and it faltered to a 5.04 ERA that was 22nd in the majors and last in the division. Bumgarner’s miserable year was a drag on their run suppression, as was Robbie Ray’s, who walked 31 batters in 31 innings before a midseason trade to the Blue Jays. Ray is gone for good, but the other four members of the 2020 Opening Day Diamondbacks rotation are projected to reprise their roles next season. They underperformed as a unit last year, and while the rotation doesn’t look exciting on paper, there’s little for the D-backs to lose by going with what they’ve got and hoping for the best.
Gallen came to the D-backs from Miami in exchange for highly regarded shortstop prospect Jazz Chisholm, and even though Chisholm will probably become a quality contributor for the Marlins, the snakes have to be happy with how the trade worked out. In his 152 major league innings thus far, Gallen has looked the part of a bona-fide Ace for this Diamondbacks rotation. He has four different pitches in his repertoire that he threw at least 17 percent of the time in 2020 (fastball, cutter, changeup, curveball), and he utilized them to strike out well over a batter an inning with a sparkling 2.75 ERA. He seems an easy choice to be their Opening Day starter, and ZiPS predicts he will be continue to be very good – if not quite as good as he was these past two seasons.
Madison Bumgarner
The Bumgarner era in Arizona has gotten off to an inauspicious start, to be sure. It looked like a legacy signing that rewarded him more for his reputation than realistic future contributions, and that initial grade looks to have been proven right thus far. His fastball velocity, which was already trending downward before coming to Arizona, plunged to new lows in 2020. His average fastball velocity in 2015, when he was still in his prime, was 92.7 mph. In 2019, it was 91.4 mph. In 2020, he was all the way down to 88.4 mph. His cutter also fell from 87.3 mph last year to 83.5 this year. This drastic year-over-year velocity drop could signal a problem with his elbow, or maybe he’s just suffering from dead arm. While he’s still only 31, he has a lot more mileage on his arm than the average 31-year-old, even if his delivery motion is pretty clean.
Aside from his worrying peripherals, the bottom-line results in 2020 just were not good. He was the second least-valuable pitcher in all of baseball among those who threw at least 40 innings, and his abysmal 6.48 ERA actually outperformed his 7.18 FIP, meaning that he was lucky not to have pitched worse. Even with all this doom and gloom, ZiPS seems to think that MadBum will be competent next year. 146 innings of sub-5 ERA ball would be a rebound campaign that provides the D-backs with needed volume of innings – assuming his arm hasn’t turned into an actual pool noodle just yet.
D-backs GM Mike Hazen must be an avid fisherman, because this is the second Marlin he’s reeled in for the Diamondbacks rotation. Smith was the biggest piece coming back to the desert in the Starling Marte trade, and he certainly has strikeout stuff that can play well in a starting role. He missed some time due to a positive COVID-19 test and never went deeper than four innings in any of his five appearances this year, but he did log 153.1 innings in 2019. When he did pitch in 2020, it was weird. It’s hard to pitch to a 2.57 ERA and be below replacement-level, but he did just that thanks to a sky-high 7.71 BB/9. He hasn’t struggled with control nearly that much in the past, so it’s probably an aberration brought on by his lack of routine and preparation this season.
How could his ERA have been so good with such a high walk rate, you ask? His BABIP (batting average on balls in play) was .100. 100! For comparison, the league average is typically around .300. In addition, his strand rate was 100%, meaning relievers didn’t allow a single one of the inherited runners Smith left on base to score. When he pitched, in other words, he was grossly lucky.
ZiPS foresees that his walk rate and BABIP will normalize next season, adding up to a mediocre yet usable one win above replacement.
Weaver and catcher Carson Kelly were the primary return pieces coming back from the Cardinals in exchange for franchise first baseman Paul Goldschmidt, and the deal doesn’t look too bad from either perspective (considering it’s doubtful they could have re-signed Goldschmidt). Both Weaver and Kelly impressed in their D-backs debuts in 2019, and both regressed in 2020. For Weaver’s part, he struck out nearly five batters for every walk he surrendered in 2019 on his way to a pristine 2.94 ERA. A good K/BB ratio is the calling card of a good pitcher – having good stuff, and the ability to control it. His strikeout rate remained pretty steady this season, but his walk rate returned closer to his career norms. This was only one of the factors which help explain his supremely disappointing follow-up campaign.
For one thing, Weaver was essentially the anti-Caleb Smith this season. Where Smith’s ERA massively outperformed his FIP, the opposite was true for Weaver. In fact, Weaver still managed to produce a positive WAR despite his lousy 6.58 ERA, while Smith graded at below replacement level even with a 2.57 ERA. Check out some handpicked year-over-year differences between Weaver’s 2019 and 2020 that help explain how that ERA got so bloated –
- K/BB: 4.93 in 2019, 3.06 in 2020
- HR/9: 0.84 in 2019, 1.73 in 2020
- BABIP: .292 in 2019, .349 in 2020
We already know he walked more batters and struck out around the same amount. Then we can see that he was more than twice as homer prone as last year, and opponents who put the ball in play were much more likely to reach base safely. LOB% (Left on base percentage) indicates 15% more of the baserunners who reached base ended up scoring. FB% (Fly ball percentage) shows that opponents hit the ball in the air nearly 11% more often when they made contact, and those fly balls that were hit went over the fence 3% more often.
Compounding all of those problems are his O-Swing% and Z-Swing%, which show that Weaver’s opponents swung at the ball less often when it was out of the strike zone, and more often when it was within the strike zone. This could be because his breaking pitches weren’t breaking as much as they were last year, or maybe he was tipping his pitches. His average pitch velocities stayed pretty consistent, so the issue at hand seems to be that his stuff was either flatter or easier to read. The good news is, that means the problems are something that Weaver and pitching coach Matt Herges can probably fix. ZiPS has faith that he’s worthy of keeping in the major league rotation, but the top-of-the-rotation potential he showed in 2019 was probably an aberration.
Kelly made his major league debut with Arizona after playing with the SK Wyverns of the KBO for four years. He was more of a durable workhorse than a flashy Ace during his time there, but the KBO is regarded as a notoriously hitter-friendly league. His modest success led to a stateside deal with the Diamondbacks that has turned out to be a pleasant surprise for the club. His 92-mph fastball doesn’t blow hitters away, but his strikeout rate looks much more impressive when compared to his stingy walk rate. He had the best ERA+ (park adjusted) on the team in 2020 and managed to provide .6 WAR despite only making five starts. Kelly could be a welcome source of dependability for the club if he can approach something similar to the 183 innings he threw in 2019, though ZiPS splits the difference by projecting a more conservative 131 innings from him.
These are the five projected opening day starters for the Diamondbacks rotation, and there’s a host of depth options behind them who will see plenty of starts of their own in 2021, including Jon Duplantier, Alex Young, Riley Smith, Corbin Martin, Taylor Clarke, and Taylor Widener. To put it another way, their list of rotation candidates includes Zac Gallen and a host of #4 or #5-type options. Still, hoping for a dead cat bounce isn’t the worst plan of action if you’re Mike Hazen.
Coming full circle, the team was a legitimate (on paper) playoff contender going into this shortened season. Poor performance in just 60 games might have had a negative impact on how the team’s chances are perceived heading into next year. If GM Mike Hazen wants to push this team back into the (on paper) playoff hunt, he’ll need to make some acquisitions – especially in the bullpen – but the Diamondbacks rotation isn’t an area he needs to spend too much time fussing over. It figures to be useful, if not league leading.
