PHOENIX — A lot of times when an organization cleans house in the front office, the incoming baseball honchos start their new regimes by taking their own brooms to the playing roster. Arizona’s Mike Hazen decided not to do that and with the Diamondbacks about to host their first postseason game in six years, the status quo never looked so good.

Last season, the Diamondbacks sported the worst ERA in baseball at 5.09. Sure, playing at Chase Field didn’t help but even in a ballpark-adjusted metric like ERA+, Arizona’s ranking (28th) still stunk.

So what Hazen did was bring back most of the same guys. Voila! Playoffs.

“I felt strongly that we had a good nucleus of players,” said emergent managing star Torey Lovullo, Hazen’s handpicked guy to lead the team. “These were guys who had different levels of success, but we felt like there was a strong enough nucleus to build around.”

It’s pretty remarkable, when you think about it. Sixty-three percent of this season’s innings for Arizona have been thrown by pitchers who appeared for them last season despite the fact that Shelby Miller‘s season ended after four starts and Archie Bradley was moved to the bullpen. Yet Arizona’s ERA (3.66) ranked third in the big leagues this season and the ERA+ was second.

Context: Last year’s team ERA was the worst in Diamondbacks history. This year’s was the best.

“Last year, pretty much everybody underachieved,” No. 1 starter Zack Greinke said. “I think the first month we didn’t pitch that good and then as the season went on, it just kept getting worse. Maybe because we didn’t see the results the first month, we tried to change. I’m not positive. This year, it kind of worked good from the beginning and kind of continued.”

Last season was an all-around disaster for the Snakes. Arizona lost 93 games while allowing the most runs (890) in baseball. The short-term future didn’t look too promising. Payroll flexibility was limited, an unfortunate reality for a club that still had Greinke on the books for five more years and $172.5 million. ESPN’s Keith Law had the minor-league system ranked dead last over the winter, quelling hopes for low-cost, in-house solutions.

That’s what Hazen inherited when he took over for Dave Stewart as Arizona’s general manager and went about assembling his first roster for Lovullo. Tearing down seemed like a viable option, perhaps even a necessary one.

However, a full-blown rebuild would almost certainly need to be kick-started by dealing Greinke, which at the very least seemed like a possible salve for the constricted payroll. It was as terrible time to do so. Greinke’s 4.37 ERA in 2016 was his highest in a decade and everything was trending the wrong way: strikeouts, walks, homers, velocity and, as it does for us all, age.

So Greinke stayed, along with most of the starting rotation, and the Diamondbacks went about improving from within.

“We were going to present some new thoughts to them about how to attack and study, pay attention to different things,” Lovullo said. “And having catcher buy-in. We felt like the combination of the guys that we had, that had had success, if we could just give them a little extra twist, a little extra turn, about how to attack hitters that they might be something real special.

“I think by mid-May, there was real buy-in for the program that we have here. It’s not a secret. Everybody is doing this. Everybody is studying and paying attention at behind-the-scenes levels that are pretty impressive. It was the buy-in that was impressive. These guys did it, had success, and it’s gotten us here today.”

Saying that Hazen stood pat isn’t quite accurate, but his moves over the winter were more about tweaking the edges of the roster to support the existing core rather than juggling the core. The exception to that was the pre-Thanksgiving trade that sent shortstop Jean Segura, outfielder Mitch Haniger and pitcher Zac Curtis to Seattle for pitcher Taijuan Walker and shortstop Ketel Marte.

Closer Fernando Rodney was signed to bring the bullpen picture into focus, and that group received a dynamic and unexpected boost when Bradley lost his bid to hold onto his rotation spot. Instead, he turned into one of baseball’s most dynamic relievers, ranking 12th among all bullpen hurlers in WAR, according to Fangraphs.com. Perhaps as important as any of this was Hazen’s decision to turn the catching chores over the veterans Jeff Mathis and Chris Iannetta.

“We felt confident that if we could fix some of those things around the edges,” Hazen said. “Some of them around defense, some around the edges that involved some different kinds of planning. Those were thing things that we felt if you could match that with the type of talent we had, it would be a regression back to what they’d been for a large portion of their career. Or, in some cases, had just shown in spurts.”

This all looks great in hindsight, but think of the pieces that Hazen needed to have fall into place. Greinke and Patrick Corbin (5.15) needed to have bounce-back seasons. Walker was coming off a two-year stretch of a 4.41 ERA and would be moving into a much harsher pitching environment at Chase Field. Robbie Ray missed bats but still needed to convert his stuff into consistent production. And then, after Miller was injured, Hazen needed another starter to emerge, while hopefully leaving the emergent Bradley to dominate in the bullpen. Unsung Zack Godley filled that role perfectly.

“We’re very fortunate that we walked into the starting pitcher group that we did,” Hazen said. “You win with starting pitching in this league and those guys have carried us all season long. We really looked at the pedigree for some of those guys, what they had done prior to this season in performance and stuff and otherwise.”

The result of all those instances of wish-casting was baseball’s third-best rotation ERA and what eventually became a solid bullpen behind them. The improvement didn’t come from any one pitcher. It came from all of them. Just look at the wOBAs allowed for every Diamondbacks pitcher who appeared in at least 10 games (with Arizona or not) both this season last season.