The Arizona Diamondbacks may face a tough decision with their starting rotation. Corbin Burnes, Zac Gallen, Merrill Kelly, and Eduardo Rodríguez are essentially locks for the top four spots, leaving the final one between three pitchers: Ryne Nelson, Brandon Pfaadt, and Jordan Montgomery. Both Pfaadt and Nelson have set a high bar in the competition based on how they've performed so far this spring.
With Montgomery struggling in his first appearance after a finger injury set him back two turns through the rotation, he's fallen behind both Nelson and Pfaadt in the competition. It's looking more likely that one of the two youngsters will claim the final rotation spot. That would likely mean a move to long relief for Montgomery. If that is indeed the case, then the question becomes who is the better choice to go with for the No. 5 spot.
"It's a very difficult discussion that we're having. I've had this very similar conversation that I had with him in Spring Training as I've had with him the past three years," Lovullo told Jack Sommers of Diamondbacks on SI.
You could make an easy argument for either Pfaadt or Nelson based on their respective 2024 performance. Pfaadt was the only starter who stayed healthy through the season, making all 32 starts and accumulating 181 innings but had occasional lapses that caused his ERA (4.71) to balloon a full run higher than his FIP (3.61). Sometimes he'd be cruising through a start and get clipped in the middle innings out of nowhere, usually when he leaves a sweeper over the plate with runners on base for home runs.
Nelson was the team's best starter down the stretch, pitching to a 3.16 ERA/3.17 FIP from July 2 to September 8. In his 12 starts, the Diamondbacks were 10-2 and he had nine quality starts. The underlying stats between the two starters in that time period slightly favor Nelson, who has an xwOBA allowed of .282 vs. Pfaadt's .309 with similar strikeout and walk rates.
It's hard to gauge based on the two's spring performance. Nelson has allowed just two runs, on solo home runs, through eight innings across three outings. Following his start against the Athletics, where he allowed just a solo home run through 3.2 innings, Nelson told reporters his fastball command was better and he's roughly three-quarters of where he needs to be.
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On the other hand, Brandon Pfaadt has put together two good outings and one bad one in his three appearances. Overall he's allowed four runs in 6.1 innings, with three of them coming in a start against the San Francisco Giants. Take away that appearance and it's one run allowed in five innings. It's enough to keep him in the race, especially with incumbency taken into account and how reliable he was at answering the bell in 2024.
Based on merit, the most qualified pitcher to take the bump should be Pfaadt. Not only does he have better underlying metrics than Nelson in the entire 2024 season, he also has the distinction of being the most reliable starter of the two. While both pitchers deserve a rotation spot on merit, one is not going to get it and will likely start the season in Triple-A Reno.