Robert Saleh

Defensive coordinator Robert Saleh of the San Francisco 49ers looks on during the NFC Divisional Round Playoff game against the Minnesota Vikings at Levi's Stadium on Saturday, Jan. 11, 2020 in Santa Clara, Calif. (Lachlan Cunningham/Getty Images/TNS)

Before Robert Saleh was a hot head-coaching candidate, the 49ers defensive coordinator was a ho-hum hire.

In 2017, the 49ers hired Saleh, then an anonymous 38-year-old position coach with zero coordinator experience, only after they couldn't land at least two coaches who were far more established: Just-fired Jaguars head coach Gus Bradley decided to become the Chargers defensive coordinator and then-Bears defensive coordinator Vic Fangio couldn't get out of his contract.

"It doesn't bother me that I wasn't the first choice," Saleh said in 2017. "We have the opportunity now. So now what? The focus is on proving (head coach) Kyle (Shanahan) right. And proving the organization right. And doing everything I can to make good on their decision."

Saleh has succeeded in doing that.

And his ascent over his four-season tenure has the 49ers bracing for his exit.

Who are the top candidates to potentially replace him?

The list includes Bradley and Fangio, if they don't retain their current jobs, and a 49ers assistant who fits the profile of Saleh, circa 2017.

DeMeco Ryans (49ers inside linebackers coach): In 2017, in Saleh's first news conference, he spoke glowingly of Ryans, then a rookie defensive quality control coach who was two seasons removed from a decorated 10-season NFL career.

"DeMeco is probably going to be a head coach one day," Saleh said. "So I'll be asking him for a job, I'm sure, not long from now. He's got that type of football I.Q. That type of presence. And is as knowledgeable as it gets."

Four years later, Ryan, who was promoted to his current position in 2018, could replace Saleh after playing a role in developing three homegrown linebackers who have exceeded expectations: Pro Bowl selection Fred Warner (third round), Dre Greenlaw, (fifth) and Azeez Al-Shaair (undrafted).

Would the 49ers hand the keys of upper-echelon defense to a 36-year-old position coach with no play-calling experience? Their experience with Saleh, the Jaguars linebackers coach from 2014-2016, could allay some fears.

On Saturday, when Shanahan was asked about possibly replacing Saleh, he indicated he would consider an in-house hire.

"I hope I don't lose Saleh," Shanahan said. "But we've got a bunch of good coaches on this staff."

Gus Bradley (Chargers defensive coordinator): A mentor of Saleh's, Bradley, 54, was the Seahawks defensive coordinator when Saleh was a Seattle assistant in 2011-12 and the Jaguars head coach during Saleh's tenure in Jacksonville.

In fact, Saleh expected to follow Bradley to the Chargers in 2017 before the 49ers reached out.

A major reason Shanahan was interested in Bradley — and Saleh — was their background in the Seahawks' 4-3 defense, which Bradley oversaw when he laid the foundation for Seattle's "Legion of Boom" units in the early 2010s.

Shanahan prefers that his teams run a 4-3 because it has given him challenges as an offensive play-caller. He views it a system that's effective in stopping the run and limiting big plays.

Under Bradley, the Chargers ranked third in the NFL in points allowed in 2017 and were fifth and eighth, respectively, in yards allowed, the next two seasons. However, the Chargers (6-9) are en route to their second straight losing season and Bradley could be part of a housecleaning that includes head coach Anthony Lynn.

Vic Fangio (Broncos head coach): All that stuff you just read about Shanahan's preference for a 4-3 defense?

Well, he'd gladly switch to a 3-4 system, which Fangio employs, if it meant landing one of the league premier defensive minds.

Fangio, 62, is a 35-year NFL veteran who spent 19 seasons as a defensive coordinator before he became Denver's head coach in 2019. His most notable work came with the 49ers from 2011-2014 when the 49ers ranked among the NFL's top five in total defense in each season and allowed the second- or third-fewest points in the league from 2011-13.

In 2017, Shanahan reached out to Fangio even before he officially became the 49ers head coach, but the Bears wouldn't let Fangio out his contract.

And it's possible Fangio won't be available in 2021. The Broncos (5-10) are 12-19 under Fangio, but he said Monday he expected to return for a third season in Denver.

"That's been the indication all along," Fangio said to Denver reporters.

Dan Quinn (former Falcons head coach): If he doesn't land another head-coaching job this offseason, Quinn, 50, will be in demand as a defensive coordinator and a reunion with Shanahan wouldn't be surprising.

Shanahan worked under Quinn for two seasons (2016-2017) as Atlanta's offensive coordinator and they developed a strong relationship. Quinn's influence helped shape certain aspects of Shanahan as a head coach.

Shanahan learned from how the personable Quinn related to players and took elements of how Atlanta prepared for the Super Bowl after the 2016 season to ready the 49ers for the game last season. Shanahan, borrowing from Quinn, has also incorporated Navy SEAL training in offseason team-building exercises.

Quinn is also a 4-3 disciple with an extensive background in Seattle's system. He took over for Bradley as the Seahawks defensive coordinator and Seattle made back-to-back Super Bowl appearances in his two seasons (2013-2014).

Quinn, who spent his first four NFL seasons (2001-2004) as a low-level 49ers assistant, was fired in October after the Falcons started 0-5. He went 43-42 in five-plus seasons.

Raheem Morris (Falcons interim head coach): Shanahan has spent six seasons working with Morris, 44, starting with the Buccaneers in 2004 when he was quality control coach and Morris was an assistant defensive backs coach.

In their first season together, Shanahan reportedly told his dad, Mike, then the Broncos head coach, that he needed to hire Morris on his staff. Mike Shanahan hired Morris as his defensive backs coach in Washington in 2012, after Morris' three seasons as Tampa Bay's head coach.

Morris had various roles on Quinn's staff since 2015 before he was promoted to defensive coordinator this season. Atlanta is 4-6 with Morris as the interim coach and have allowed 20.9 points a game after surrendering 32.2 points in their 0-5 start.

Eric Branch covers the 49ers for The San Francisco Chronicle. Email: ebranch@sfchronicle.com Twitter: @Eric_Branch

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