Last time the Diamondbacks dished out a multi-year contract to a 30-year-old left-handed starter, it did not go well.

It was December of 2019. The Diamondbacks were coming off a promising 85-win season. After dealing Zack Greinke to the Houston Astros earlier in the year, they were looking for another starter to lead a rotation that included Robbie Ray, Luke Weaver and a pair of inexperienced righties: Zac Gallen and Merrill Kelly.

Long-time San Francisco Giants ace Madison Bumgarner, a part-time Phoenix resident for more than a decade, was their man. The Diamondbacks signed Bumgarner to a five-year, $85 million contract. He reportedly came at a discounted rate because of his desire to live in the area.

BREAKING: Left-hander Madison Bumgarner and the Arizona Diamondbacks are nearing a five-year deal, sources familiar with the agreement tell ESPN.

— Jeff Passan (@JeffPassan) December 15, 2019

Roughly 3 1/2 years later, Bumgarner’s tenure with the Diamondbacks came to an early conclusion. After seeing him post a 6.50 ERA in the second half of the 2022 season and a 10.26 ERA in four starts to start 2023, the Diamondbacks designated the veteran lefty for assignment in April. In the process, they ate nearly $35 million — nearly half of Bumgarner’s deal.

When the decision was made, Diamondbacks general manager Mike Hazen was asked how he makes sense of Bumgarner’s sudden decline.

“We’ve all lost a lot of sleep over trying to answer that question,” Hazen told reporters. “I haven’t answered it yet, and we’re here today. I don’t have the answer for you. It just didn’t work out.”

In his shortened Diamondbacks career, Bumgarner went 15-32 in 69 starts with a 5.23 ERA and 1.38 WHIP. More than falling short of his $85 million contract, he was one of the worst pitchers in baseball from 2020-23. His signing was the kind of mistake the franchise cannot afford to make again.

In spite of the results of Bumgarner’s deal, Diamondbacks general manager Mike Hazen has been adamant that one bad contract would not keep him from being aggressive in free agency.

“Not at all,” Hazen said at the GM meetings last month. “If it did, I’ll go find another job.”

Hazen followed through on that promise this week by reportedly signing another 30-year-old lefty, Eduardo Rodriguez, to a four-year, $80 million contract. The deal also includes a vesting option for a fifth year that would bring the total value to around $100 million. The Diamondbacks have not confirmed the deal.

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Rodriguez is coming off one of the better seasons of his eight-year, major-league career. In 152 2/3 innings with the Detroit Tigers in 2023, he went 13-9 with a 3.30 ERA, 1.15 WHIP, 23 percent strikeout rate and 7.7 percent walk rate. He was on track to make his first All-Star team before being sidelined with a finger injury in late May.

The Diamondbacks’ new lefty will join a rotation that includes Zac Gallen, Merrill Kelly and Brandon Pfaadt. With Gallen and Kelly already being viewed as one of the better duos in baseball in 2023 and Pfaadt showing promise in the second half, the Diamondbacks have the makings of a top-10 starting rotation in baseball.

Of course, for that to happen, they will need Rodriguez to continue to be productive, something Bumgarner was largely unable to do the moment he donned a Diamondbacks jersey.

It would be absurd to expect Rodriguez to suffer the same fate as Bumgarner simply because he happens to be a 30-year-old lefty at time of signing; few pitchers drop off so quickly. Still, Bumgarner was pitching well when the Diamondbacks signed him, and it would be naive to say that such a steep decline should have been seen from miles away. What happened to Bumgarner can and will happen to others.

Here is a look at how Rodriguez and Bumgarner compared in their seasons prior to signing with Arizona:

Bumgarner had better strikeout and walk rates and threw more innings in his walk year, but Rodriguez had a much better ERA.

Projecting Rodriguez for a 3.30 ERA moving forward would be bullish — his 4.04 expected ERA, 3.66 FIP and 4.06 xFIP all indicate that he over-performed to some degree this year — but he was still worth 3.0 fWAR despite missing more than a month with injury.

For more context, we can expand our search to three seasons prior to signing with Arizona:

Bumgarner has the edge nearly across the board, but it is worth noting that Rodriguez’s FIP (3.70) was actually better than Bumgarner’s (3.94) in this three-year span.

What Rodriguez does have going for him compared to Bumgarner is that his final year before signing with Arizona was possibly his best ever. Bumgarner, on the other hand, showed some regression in his final year in San Francisco. His 3.90 ERA in 2019 was the highest of his big-league career at the time.

All this is to say: While Rodriguez showed less regression in his walk year, he and Bumgarner are fairly comparable in terms of how they performed before signing with Arizona. Anyone looking for a concrete reason that Rodriguez will age better than Bumgarner is not going to find it in those stats. They had similar numbers, and they received similar contracts.

There is one clear-cut difference between the two, however: innings.

Prior to signing with the Diamondbacks, Bumgarner had already logged 1,948.1 career innings, including the playoffs. When asked about Bumgarner’s struggles, Diamondbacks pitching coach Brent Strom referenced mileage on the arm as a significant factor.

After eight seasons as a starter in the majors, Rodriguez is relatively fresh. He has thrown 1,123 innings, including the playoffs, in his major-league career. That is roughly half the number of innings that Bumgarner had at the same age.

Granted, Bumgarner’s durability was one of his greatest strengths as a pitcher. The fact that Rodriguez has only reached 160 innings in a season once is not exactly a selling point. Nonetheless, the difference in career innings is massive, and it suggests that the quality of Rodriguez’s innings could be better moving forward, even if he is never able to deliver a full season of them.

Rodriguez has missed time in all but one season in the majors, including leaving the Tigers for about two months to work through marital issues in 2022. However, he has avoided any serious arm injuries.

So, what should be expected from Rodriguez moving forward? Now on the wrong side of 30, some level of decline seems inevitable.

Public projection systems such as ZiPS and STEAMER peg Rodriguez for an ERA in the mid-to-high threes in 2024, increasing steadily each year thereafter. Rodriguez might not post an ERA in the low threes again.

Nonetheless, Rodriguez has been fairly consistent throughout his career. This is not a case of a pitcher who was once dominant and is now wearing down. His FIP has ranged from 3.32 to 4.43 in his career, and it was a healthy 3.66 in 2023. Similarly, his xERA has always fallen between 3.47 and 4.28, and it was 4.04 this past season.

Rodriguez does not generate enough whiffs to be a frontline starter, but, in Arizona, he does not have to be. And that brings us to a final comparison point with Bumgarner.

The Diamondbacks are not banking on Rodriguez being their No. 1 starter, as they were with Bumgarner. Gallen and Kelly are under contract for two more seasons, and Pfaadt is emerging. Rodriguez projects as a No. 3 starting pitcher, and that is precisely what he will be.

Granted, dedicating $20 million per year to a mid-rotation starter is a hefty investment that carries undeniable risk. It seems possible — perhaps likely — that Rodriguez will make more than his performance warrants in the not-too-distant future. That could be alright, as long as Rodriguez’s performance does not drop off as quickly and dramatically as Bumgarner’s did. On paper, there are good reasons to believe that it won’t.

The cost was significant, but the Diamondbacks’ reported four-year, $80 million contract with Rodriguez appears to be a fair deal that fills an immediate area of need. Time will tell if it works out.

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