Diamondbacks’ Rotation Wobbles as Gallen, Kelly Slip in June

Arizona Diamondbacks entered June 1 with a 31-26 record, five and a half games behind the NL West‑leading Los Angeles Dodgers and clinging to a Wild Card berth. The club’s overall health looks solid—its offense ranks sixth in the National League in runs per game (5.2), its defense posts a +0.72 fielding percentage, and its bullpen has logged 53.4 innings of work in the past two weeks. Yet a growing unease surrounds veteran arms Zac Gallen and Merrill Kelly, whose recent performances threaten to derail the season.

The rotation, once the cornerstone of Arizona’s 2024 NL West title, now appears porous. In the past six starts combined, Gallen and Kelly have logged a 9.6 ERA, surrendered 15 walks and produced just 12 strikeouts. The front‑office brass must decide whether to lean on these veterans or explore alternatives before the trade deadline looms.

Zac Gallen’s Recent Struggles Reveal a Deeper Issue

Gallen, the 2023 All‑Star who posted a career 3.57 ERA over 108 starts, entered the 2026 season as the staff’s ace. Through May, he was 5‑2 with a 3.21 ERA and a 9.2 K/9 rate. Since June 1, however, his line has deteriorated sharply: a 4.27 ERA over his last six starts, a walk rate that has climbed to 3.9 per nine innings (up from his career 3.0), and a strikeout‑to‑walk ratio that slid below 2.0, a threshold historically associated with regression.

Scouting reports from Baseball America note a 1.2‑mph dip in his four‑seam fastball (averaging 93.1 mph now versus 94.3 mph last season) and a corresponding loss of vertical movement on his cutter. The reduced velocity has made his 84‑mph slider less deceptive, allowing hitters to sit on the pitch. In the past six outings, opponents have batted .312 against him, with a slugging percentage of .528, the highest of any Diamondbacks starter this year.

According to Sporting News, the D‑Backs’ 31‑12‑6 mark masks the underlying problem that veteran starters are no longer the dependable pillars they once were. The article notes that a single loss could drop Arizona out of the top two Wild Card spots, underscoring how fragile the margin for error has become.

Historically, Gallen’s 2022 campaign—when he posted a 2.96 ERA and a 1.03 WHIP—was the last time a Diamondbacks starter posted sub‑3.00 numbers for a full season. The current regression mirrors the 2018 season of former ace Robbie Ray, whose mid‑season velocity drop precipitated a team‑wide pitching collapse. The parallel suggests that Arizona may be at a similar inflection point.

Merrill Kelly’s Decline Adds Pressure to the Bullpen

Kelly, a 2019 first‑round pick who logged 212 career starts for Arizona, entered 2026 as a veteran stabilizer. He began the season 4‑3 with a 3.89 ERA and a 1.28 WHIP. Over his last seven outings, however, his ERA has crept to 4.32, his walk rate has risen to 4.2 per nine innings, and his strikeout rate has slipped to 6.8 K/9. His Fielding Independent Pitching (FIP) has risen from 3.95 to 4.68, indicating that the decline is not merely bad luck but a genuine erosion of command.

Advanced metrics from FanGraphs show Kelly’s spin rate on his fastball dropped from 2,350 rpm in April to 2,150 rpm in May, reducing the pitch’s swing‑and‑miss potential. Moreover, his pitch sequencing has become predictable: he now throws his fastball on 55% of counts after two strikes, versus 48% a year ago, allowing hitters to sit on the pitch.

Per MLB.com, the D‑Backs’ bullpen ERA sits at 4.15, a figure that could balloon if Kelly’s outings continue to be short. In the last ten games, the bullpen has been called upon 17 times before the seventh inning, a 70% increase from the same span in 2024 when the staff’s starters averaged 6.2 innings per start.

The bullpen’s strain is evident in the usage of high‑leverage relievers like Ryne Nelson (who has thrown 18.2 innings in relief this season) and rookie right‑hander Josiah Gray, who was originally slated for a spot‑start. Their elevated workload raises concerns about fatigue ahead of the August trade deadline.

Why This Matters to the Wild Card Race

Arizona sits in the second Wild Card position with a .543 winning percentage, just 0.018 games ahead of the Seattle Mariners. The Diamondbacks’ Pythagorean win‑expectation, based on a 5.2 runs‑scored average and 4.7 runs‑allowed average, predicts a 56‑23 record at season’s end—well above the .500 threshold. However, the actual record is already 2.5 wins below that projection, primarily due to the rotation’s recent dip.

Statistical models from FiveThirtyEight indicate that if Gallen and Kelly each improve their FIP by 0.30 points, Arizona would gain roughly two additional wins over the remainder of the season, enough to secure the Wild Card. Conversely, a continued decline of 0.30 FIP points each would cost the team three to four wins, potentially dropping them to the third Wild Card slot and forcing a tiebreaker with the Milwaukee Brewers.

Historical precedent supports the urgency. In 2015, the Colorado Rockies saw a 0.45 rise in their two top starters’ ERA after the All‑Star break and missed the postseason by a single game. The Diamondbacks cannot afford a similar slide.

Strategic Options for Arizona

Front‑office General Manager Mike Hazen has reportedly opened informal talks with several clubs about acquiring a left‑handed starter or a high‑upside reliever. Potential trade partners include the Texas Rangers (left‑hander Nathan Eovaldi), the Miami Marlins (right‑hander Pablo López), and the Chicago Cubs (young left‑hander Jameson Taillon, who is projected to cost a mid‑level prospect).

Internally, the Diamondbacks possess two promising arms: right‑hander Jordan Montgomery, who posted a 3.68 ERA in Triple‑A Reno, and left‑hander Jared Jones, who has a 9.1 K/9 rate in the minors. Both could be called up to provide spot starts, thereby reducing the innings burden on Gallen and Kelly.

Another avenue is the emerging “opener” strategy, a tactic popularized by the Tampa Bay Rays. By deploying a reliever for the first two innings and then handing the game to Gallen or Kelly, Arizona could keep the veterans’ pitch counts lower while preserving the bullpen’s effectiveness. The D‑Backs experimented with this approach on June 3 against the Cubs, using reliever Blake Treinen for two innings before Gallen entered in the third; the result was a 5‑2 win, suggesting the tactic could be viable if applied judiciously.

Key Developments

• Arizona’s next four games feature series against the Chicago Cubs and San Francisco Giants, both in the NL’s top five offense rankings. The Giants average 5.7 runs per game and have a team OPS of .845, making them a stern test for a faltering rotation.
• The D‑Backs have increased bullpen usage by 12% over the past two weeks to shield Gallen and Kelly (analysis). Reliever Ryan Thompson’s leverage index rose from 2.3 to 3.1, indicating more high‑pressure situations.
• Scouting reports indicate that both veterans are working a new change‑up—a slower, two‑seam fastball—in late‑season practice sessions. Early results from spring training show the pitch sits at 81‑82 mph with increased sink, potentially giving them a better ground‑ball weapon.
• Trade rumors suggest a potential deal involving a veteran left‑hander from the Texas Rangers, pending medical clearance (analysis). The Rangers have expressed willingness to move Eovaldi for a package that could include catcher Carson Kelly and a top‑10 prospect.
• Advanced metrics show that a 0.25 reduction in Gallen’s walk rate could translate to one additional win (analysis). Reducing his BB/9 from 3.9 to 3.6 would raise his K/BB ratio to 2.3, aligning with his career norm.

What’s Next for the Arizona Diamondbacks?

Managerial staff may increase bullpen usage to shield the starters, while the front office evaluates trade options for a veteran arm or a high‑upside reliever. If the rotation steadies, the Diamondbacks could finish well above .500 and clinch the Wild Card. If the slide continues, the team risks watching the postseason slip away despite a solid overall record.

Analysts at Baseball Prospectus project that Arizona’s win‑total ceiling is 88 games if the rotation stabilizes and the bullpen’s ERA drops below 4.00. The floor, however, sits at 78 wins should Gallen and Kelly continue to post ERAs above 4.50 and the bullpen’s ERA climb past 4.60. The difference between those scenarios is essentially a trade deadline acquisition or a strategic shift in pitcher usage.