When the Diamondbacks acquired Luke Weaver from the St. Louis Cardinals in 2019, he was not far removed from being one of baseball’s best pitching prospects.

Ranked 68th on MLB Pipeline’s 2017 pre-season prospect rankings, Weaver first broke into the majors in August of 2016 at the age of 22. Earlier that year, he posted a ridiculous 1.30 ERA, 0.93 WHIP and 7.67 strikeout-to-walk ratio in 83 innings between the Cardinals’ Double-A and Triple-A affiliates.

Weaver was far from a sure thing; no prospect is. Scouts suspected he would need to develop a more formidable third pitch to succeed at the highest level. Nonetheless, he had a lot of the attributes you look for in a starting pitcher: an effortless, repeatable delivery; good command; a lethal two-pitch mix that generates whiffs; and feel for a curveball and cutter that gave him a fighting chance at becoming a frontline starter.

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Fast-forward to 2022. Weaver didn’t make his team’s starting rotation out of spring training, and he has a 7.71 ERA in 16 innings as a reliever. On Monday, the Diamondbacks dealt him to the Kansas City Royals in exchange for 26-year-old platoon third baseman Emmanuel Rivera. It is probably safe to say this wasn’t the ending that Weaver, the Diamondbacks’ front office or the fanbase had in mind. 

Weaver’s stint in Arizona started off well, though. In 12 starts in 2019, Weaver went 4-3 with a 2.94 ERA, 1.07 WHIP and 69 strikeouts against just 14 walks in 64 ⅓ innings. He was using his cutter more often while still throwing the occasional curveball. Combined with his fastball and changeup, his pitch mix proved lethal — for about two months.

On May 27, 2019, Weaver was diagnosed with what was originally deemed a forearm strain. A few days later, the Diamondbacks announced that Weaver had a flexor pronator strain and a mild sprain of his ulnar-collateral ligament. After visits with four physicians, Weaver chose rehab over Tommy John surgery.

Remarkably, he pitched again later in the season, facing six batters against the San Diego Padres in late September and retiring all six. Unfortunately, he hasn’t been the same since.

After reportedly being restored to full health for the 2020 season, Weaver struggled mightily to the tune of a 1-9 record and 6.58 ERA in 12 starts. His changeup remained effective, albeit with less vertical depth than the year prior, but opposing hitters batted over .300 with a slugging percentage near .600 against both his fastball and cutter.

Weaver was better in 2021, finishing the year with a 4.25 ERA and 1.19 WHIP, but he still wasn’t the same pitcher from 2019. Rather than adding to his pitch mix, Weaver simplified it, throwing fastballs or changeups 92 percent of the time. He improved overall, but his changeup generated fewer whiffs. What’s worse, he missed more than three months due to a shoulder issue.

Ultimately, after spending three-and-a-half seasons as a Diamondback, Weaver logged just 198 ⅓ innings in Sedona red from 2019 to 2022. Merrill Kelly is on pace to throw more innings this year alone.

When asked about Weaver’s progression in Arizona, general manager Mike Hazen acknowledged it’s hard to pinpoint what went wrong.

“I’m not exactly sure,” Hazen said. “He was electric, and [then] the injury happened. And then the second injury happened, and we kind of just got put behind the eight ball and then time kind of time kind of ticked away on us.

“It’s just unfortunate. He’s got a great arm, and we love who he is as a competitor and as a player.”

In some ways, it’s remarkable the Diamondbacks found a suitor for Weaver in light of his 7.71 ERA. It is not out of the question that the team could have non-tendered him this winter. Granted, Weaver’s peripherals — namely, his 24.1 percent strikeout rate and 6.3 percent walk rate — suggest he’s been better than his inflated ERA would have us believe.

In return, Rivera fills a need for the D-backs as a right-handed hitting third baseman. Hazen said Rivera will get a lot of opportunity against lefties, against whom he is slashing .288/.347/.546 this season.

As for Weaver, he joins a growing list of young Diamondbacks pitchers from the last decade who have struggled to progress and stay consistent. Other cases include Robbie Ray, Zack Godley, Braden Shipley, Jon Duplantier, Archie Bradley, Shelby Miller and others.

Over the last decade, four Diamondbacks pitchers have produced three or more fWAR in a single season: Godley, Ray, Patrick Corbin and Zack Greinke.

Obviously, Greinke was successful long before he became a Diamondback. Of the other three, Godley and Ray each had one breakout season — 2017 — but took a turn for the worse after. Corbin struggled after returning from Tommy John surgery, but that isn’t uncommon. He was excellent before the procedure in 2013 and he had his best season ever in 2018.

On the whole, however, there is no getting around the fact that the Diamondbacks have a shaky track record developing young big-league starters. To be fair, developing starting pitchers is very, very difficult, and the D-backs are far from the only team in baseball that has struggled in that regard. It’s also worth noting that Merrill Kelly is on pace for a three-plus win season in 2022, and the team deserves some credit for his success since coming over from Korea.

As far as Weaver is concerned, the fact that the Diamondbacks originally acquired him in exchange for Paul Goldschmidt does not improve the optics. In fact, with Andrew Young claimed in the minor league Rule 5 draft and Weaver dealt to Kansas City, Carson Kelly is now the only remnant of the deal that sent out the Diamondbacks’ star first baseman in 2019. While Kelly has looked better in the last month, he has struggled overall this season at the plate, slashing .215/.272/.367 in 173 plate appearances.

Past trades do not mean much now, though. All eyes in Arizona are set on the team’s bright future  — a future that relies on the team’s ability to develop its plethora of pitching prospects that are at the upper levels of the minors. 

It would be unrealistic to expect all of them to turn into consistent, productive big-league starters, but the Diamondbacks have a lot to gain if two or three could get to that point. Doing so will require turning what has been one of the organization’s greatest weaknesses into a strength.

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