Offseason Deep Dives: Ryne Nelson

The Diamondbacks agreed to terms with old friend Merrill Kelly over the weekend, giving him a two-year, $40 million contract to return to the desert after they had dealt him away at the trade deadline. While Kelly should provide some much-needed stability to Arizona's rotation, the biggest key among the team's starting five might be the progression of Ryne Nelson.

Nelson began the 2025 campaign in a long-relief role and made three spot starts through the end of May. He was part of the Diamondbacks' rotation for good from June on and collected a 3.38 ERA, 1.07 WHIP, 21.6 percent strikeout rate and 6.7 percent walk rate covering 20 starts over the final four months of the regular season.

Nelson finished second to Kelly among Diamondbacks hurlers in 2025 in ERA (3.39) and WHIP (1.07). That was after Nelson had collected a 4.24 ERA and 1.25 WHIP in 2024, but did the 27-year-old actually improve in his third full season in the big leagues?

Most of the ERA indicators say "no."

Nelson's ERA improvement was fueled in large part by a .252 BABIP, which was the ninth-lowest mark in baseball among the 70 pitchers to throw at least 150 innings. That was a 56-point improvement from his .308 BABIP in 2024, and it came about even as Nelson gave up louder contact in 2025, with his hard-hit rate going from 41.6 percent to 45.5 percent and average exit velocity going from 90.1 mph to 91.4 mph. (His barrel rate held steady at 7.5 percent.)

Nelson is something of a throwback in how much he leans on his fastball. Only Freddy Peralta, Kevin Gausman and Robbie Ray threw more total four-seamers than Nelson this past season, and no starting pitcher carried a higher usage rate of their four-seamer than Nelson's 61.9 percent mark. You can't really blame him, because it's a fantastic pitch.

As you can see from the image above taken from Baseball Savant, Nelson's four-seamer was the best in baseball among all heaters in terms of run value. Only Tarik Skubal's changeup — at 25 — had a higher run value ranking among all pitches. It also stood alone at the top when it came to Stuff+ for pitchers with at least 150 innings under their belt, as seen below in the image taken from FanGraphs:

Nelson's average four-seamer velocity of 95.7 mph last season was good, not great, ranking in the 72nd percentile. What makes the pitch special is its induced vertical break, combined with the right-hander's extension.

The above rankings from Baseball Savant show pitchers that threw at least 1,000 pitches this past season, and only Nick Pivetta's four-seamer produced more induced vertical break than Nelson's among that group. When you add in an extension that ranked in the 87th percentile, it creates "hop," or "rise," resulting in Nelson's perceived velocity on the pitch coming in at 96.7 mph, a full mph more than the actual velocity.

Unfortunately, Nelson is still looking for a pitch he can trust beyond his four-seamer. He threw five different offerings in 2025, but after the 61.9 percent usage on his heater, the next-highest was his cutter, at just 12.2 percent. Nelson's cutter, slider and curveball were all in the 10.9 percent to 12.2 percent usage range, while a seldom-used changeup came in at just 2.8 percent.

Of Nelson's four non-four-seamer pitches, his slider was the only one to net a positive run value, at plus-1. The pitch produced a .231 xwOBA and 33.6 percent whiff rate. Dating back to when he broke into the big leagues in 2022, Nelson has had a positive run value on just five of 19 non-four-seamer pitches. The slider is responsible for three of those, and they came in each of the last three seasons.

It would seem, then, that the slider is Nelson's best chance to develop a viable second pitch. That's particularly important because Nelson had pretty significant reverse splits in 2025, with lefty bats posting a .565 OPS and .251 wOBA, while righty bats had a .729 OPS and .312 wOBA. Twelve of the 17 home runs Nelson served up were hit by right-handed batters. The slider was Nelson's most-used secondary pitch versus righties, and it fared well with a .254 xWOBA. Righties destroyed his cutter, though, with a .400 xwOBA and .633 slugging percentage.

Nelson is headed into his age-28 season, will enter spring training locked into a rotation spot for the first time, has good control and possesses an elite pitch. That's a good starting point, but is it enough? The Diamondbacks had a pretty good defense in 2025 from an Outs Above Average perspective at plus-9, which ranked 12th. They were just 23rd in Defensive Runs Saved, though, and with Ketel Marte's and Alek Thomas' names popping up in trade rumors, their defense might get worse. That would be bad news for Nelson, who has the ball put in play against him a lot.

A repeat of 2025 feels like a best-case scenario for Nelson, but it's more likely that we're looking at a high-3s ERA, a WHIP around 1.20 and a substandard strikeout rate. That makes the right-hander a perfectly fine pick around 250 where he's been going in NFBC drafts, but there's a better chance he takes a step back in 2026 rather than a step forward.