What’s the biggest problem for the Arizona Diamondbacks heading into the 2026 MLB season?

The bullpen, which has long been a massive liability, has evolved into a structural flaw. Not a slump, not bad luck, just a roster-building issue. Not only is the injured bullpen facing a ton of question marks, but Arizona is preparing to open the season without a single left-handed reliever. That is a reality that clashes with how modern baseball is managed. Matchups aren’t optional anymore; they’re foundational, and the Diamondbacks are at a disadvantage before the first pitch.

The absence looms larger when you consider who isn’t available. A.J. Puk and Justin Martinez, two high-leverage arms, are sidelined after Tommy John surgery, removing swing-and-miss depth from late innings. That puts pressure on a group already stretched thin. Then there’s Paul Sewald. Once a stabilizing force, his recent years have trended downward with declining strikeout rates and rising hard-contact numbers. The Diamondbacks are betting on a rebound, but hope isn’t a strategy when protecting one-run leads in the seventh inning.

Internally, the plan hinges on upside. Joe Ross and Jonathan Loáisiga are the wild cards, arms with talent but inconsistent recent track records. If they hit, Arizona can tread water. If they don’t, the bullpen becomes a nightly gamble.

For D-backs fans, the concern is familiar. Arizona’s lineup can compete, and the rotation can stabilize, but games are decided in the margins. In years past, this problem has haunted them, and right now, the Diamondbacks are still unequipped to control them. Until Arizona adds balance, specifically left-handed relief and reliable late-inning production, every lead will feel temporary.