When the Arizona Diamondbacks entered September in first place in the National League West, they seemed certain of reaching the postseason in some capacity, be it as the division champs or as a wild card for the second consecutive October.

Some 22 days into September, with a week remaining in the regular season, the D-Backs have nonexistent playoff odds. A loss (or wins by other teams) on Sunday will cinch their mathematical elimination from postseason contention. The Diamondbacks have gone 5-15 in September, all the while being outscored by nearly two runs per game.

What exactly has fueled the D-Backs' cratering? A team-wide meltdown.

MLB: Los Angeles Dodgers at Arizona Diamondbacks
Paul Goldschmidt is the only Diamondback hitting. USATSI

The Diamondbacks' lineup has struggled to some extent all season. They entered Sunday ranked last in OPS+ and 27th in wRC+ (FanGraphs' park-adjusted catch-all metric). Those numbers are weighed down in part by a September that has seen Arizona average 3.65 runs per game. For perspective, the worst offense in baseball this season belongs to the Miami Marlins, who are averaging 3.67 runs per game. No other team is below 3.75. The D-Backs, then, have effectively had the majors' least-fruitful offense for nearly a month.

Arizona has given at least 30 plate appearances to 10 players in September. Only two of them have an OPS above .750 -- and one of those, Ketel Marte, is an out away from falling below the mark. Four of the team's top five in plate appearances this month have an OPS of .702 or worse, and that includes trade deadline acquisition Eduardo Escobar. The D-Backs' big offseason add, Steven Souza Jr., has a .376 OPS on the month -- he's hitting .070/.259/.116.

The only Snake who seems immune to the team-wide slump is Paul Goldschmidt. He's hit 318/.400/.545 in vain. Take the best non-Goldschmidt offering in each of the triple-slash categories and the resulting line is .254/.304/.457 -- or about what Mike Moustakas has done this season. With due respect to Moustakas, a fine player, that's not what you want.

MLB: Atlanta Braves at Arizona Diamondbacks
Brad Boxberger blew three saves in his first four outings of the month. USATSI

It hasn't been only the lineup causing problems. The Diamondbacks pitching and defense have yielded 5.45 runs per September game. The Boston Red Sox, baseball's best offense, score 5.32 runs per game.

Robbie Ray and Patrick Corbin have done their jobs. Zack Greinke (14 earned runs in 25 ⅔ innings) and Zack Godley (13 in 12 ⅔ innings) have not. Let's focus in on two other pitchers battling tough months: Matt Andriese and Brad Boxberger.

Andriese had given up six runs in his first 13 ⅔ innings with the D-Backs after coming over in a midseason trade from the Tampa Bay Rays. He then made horrendous back-to-back appearances, including a spot-start, that saw him permit 12 earned runs in 3 ⅓ innings.

Boxberger, meanwhile, has since been removed from the closer's gig after beginning the month with three losses in his first four appearances. Few things can swing a team's fortune like their performance in high-leverage situations. Unfortunately, for the Diamondbacks, those three losses pushed them closer to the edge of non-contention.

The Diamondbacks defense may deserve some blame for their slide, too. They've already allowed nine baserunners to reach via error (tied for their most in any month), and they've contributed to a .329 batting average on balls in play. Previously, Arizona had allowed a BABIP over .300 just once this season -- .301 in July.

As such, September's collapse has been a total team effort. Perhaps that's heartening in a sense -- there's no wondering what would've happened if the offense had consisted of more than Goldschmidt -- but it also suggests that the Diamondbacks will have to reconsider their roster this winter.

And that no matter what the D-Backs do through August next season, fans will worry that a repeat is in order.