Entering the 2024 season, it looked like the Diamondbacks‘ biggest strength was going to be their starting rotation.

They already had a potent 1-2 punch in Zac Gallen and Merrill Kelly. Brandon Pfaadt seemed poised for a leap forward after an impressive showing in the playoffs. They signed two of the best free agent starters available in left-handers Eduardo Rodriguez and Jordan Montgomery. On paper, it looked like a top-10 rotation in baseball; arguably top five.

But so far in 2024, the Diamondbacks’ rotation has been their weakest link.

That fact was on display during the team’s Labor Day weekend series loss to the Dodgers, in which their starters posted a combined 6.75 ERA and 1.90 WHIP in 20 innings of work.

If we zoom out to the entire home stand against the Dodgers and New York Mets, D-backs starters have posted a 6.94 ERA and 1.68 WHIP in their past seven games. It’s no coincidence that the team went just 2-5 in that span.

“We got to be better,” Diamondbacks manager Torey Lovullo said of his pitchers after Monday’s loss. “I believe in our pitching staff. I believe in our starters. I know that we’ve taken some lumps as a starting rotation. I know where we sit overall, but I believe in our guys and I know they’re better than what we’ve seen here and they’ll be the first ones to tell you that.

“We’re going to be fine, but we got to go out there and prep and execute. A win for me is doing the work. That’s the most important thing, getting the work done and doing it right.”

In a pivotal series finale on Monday, Rodriguez managed to keep the Diamondbacks in the game by allowing only three runs, but an elevated pitch count led to an early hook after four innings. The outing brought Rodriguez’s season ERA to 5.55.

Granted, Rodriguez’s subpar performance was far from the only reason that the Diamondbacks lost.

Neither Montgomery, who was recently moved to the bullpen because of how much he struggled in the rotation, nor right-hander Slade Cecconi, who was called up as bullpen depth when rosters expanded on Sunday, could hold the Dodgers at bay in relief. The Diamondbacks made plenty of other mistakes too, such as when Jake McCarthy tried (and failed) to steal second while slugger Josh Bell had a 3-0 count in the sixth.

Still, while Rodriguez’s performance was not detrimental, it also did not push the Diamondbacks toward victory. Such outings have been the norm for the D-backs for much of the season.

Analyzing Diamondbacks’ starting rotation struggles

So far in 2024, Diamondbacks starters have combined for a 4.85 ERA. That’s the third-worst mark in baseball, ahead of only the Miami Marlins and Colorado Rockies.

Granted, the Diamondbacks were without several key starters for much of the year. Rodriguez and Kelly each missed about four months with shoulder injuries. Gallen missed roughly a month with a hamstring injury.

But on Aug. 11, the Diamondbacks’ rotation was finally made whole. And ironically, it’s gotten worse, not better. Since Aug. 11, D-backs starters have a combined 5.56 ERA.

One might assume that the combined ERA is only high because one or two starters have been particularly poor. But that’s not the case. In that span, Gallen, Kelly, Pfaadt and Rodriguez all have ERAs north of 5.00. Only one Diamondbacks starter does not. That would be Ryne Nelson, who has a pristine 2.41 ERA in that span.

Entering the year, Nelson probably would’ve landed seventh on a D-backs starting pitcher depth chart, but he has been a revelation over the past two months. Since July 1, he has a 2.77 ERA and is averaging over six innings per start.

Clearly, Nelson’s recent performance isn’t concerning. But what about the others?

First, it’s worth noting that several underlying stats suggest that D-backs starters at large have pitched better than the aforementioned 5.56 ERA would lead us to believe. For example, the unit entered Monday with a collective 4.19 FIP and 3.74 xFIP since Aug. 11. The former is effectively average; the latter is better than average.

Looking closer, it’s not all that hard to understand why ERA estimators such as FIP and xFIP differ so much from actual ERA. Entering play on Monday, D-backs starters had solid strikeout and walk rates of 22.6 percent and 6.4 percent, respectively, since Aug. 11. Generally, if pitchers maintain strikeout and walk rates in those ranges, they’re going to be reasonably successful.

Of course, we can only learn so much from team-wide stats. Here’s a player-by-player look at these numbers, with the league averages for starting pitchers below for reference. (Note that Rodriguez’s FIP and xFIP are only current as of the start of play on Monday.)

It doesn’t take long to realize that Kelly’s underlying stats are, by far, the worst of the group.

In four starts since his return from the injured list on Aug. 11, Kelly has a 6.75 ERA and has struck out just 12.2 percent of opposing hitters while walking 8.2 percent.

On Saturday, he allowed six runs (four earned) on 10 hits against the Dodgers in 5 1/3 innings. After the game, he said that all signs point to one central problem.

“The game in general can be summed up by just missed locations,” Kelly said. “Since I’ve come back, I would say that’s probably the biggest hindrance so far.”

Kelly has never had overpowering stuff, so location is particularly important. His velocity was down slightly in his first several starts off the IL, but it was effectively right in line with where it was last year on Saturday.

Kelly was encouraged somewhat by the velocity uptick, but his persistent lack of command made it feel less significant.

“Obviously, that all means jacksh-t if I can’t throw it where I want to,” Kelly said after the game.

Perhaps the best argument for Kelly to turn things around is the fact that he has still only made four starts since coming off the IL. Often times, it can take a while for a starter to find a rhythm after an extended IL stint.

Of course, the fact that Kelly’s recent peripherals are noticeably worse than other D-backs starters does not mean that Gallen, Pfaadt and Rodriguez have been perfect, or that bad luck is the sole explanation for their struggles.

Gallen’s fastball has not been nearly as effective this year. Pfaadt continues to be bitten by the so-called big inning, something that underlying stats may not fully capture. Rodriguez has struggled to get opposing teams to swing and miss consistently, while also having the benefit of facing two of the league’s worst offenses in the Colorado Rockies and Miami Marlins in his past four starts.

Ultimately, given the talent that’s there, it seems realistic to expect the Diamondbacks’ rotation to be better the rest of the way. But the margin for error has gotten slimmer.

After their series loss to the Dodgers, the Diamondbacks are now six games out of the NL West, with the Dodgers holding the tiebreaker. Meanwhile, their cushion in the wild card race is down to three games over the New York Mets.

October is still a month away. There is still a lot of baseball left to be played.

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Top photo: Owen Ziliak/The Republic