Calif. — In the days after losing a low-scoring slugfest against the Los Angeles Rams, San Francisco 49ers defensive end Nick Bosa could finally see the light at the end of the tunnel.

Through the first 14 games under coordinator Nick Sorensen, the defense had been inconsistent. But that Dec. 12 matchup in which they had limited the Rams to 12 points and 4.6 yards per play left Bosa encouraged about the defense’s trajectory.

Bosa was so heartened by the defensive progress that he suggested Sorensen and much of the defense would and should remain the same in 2025, even going so far as to say “we have the right people in terms of coaches and players.”

But after the 49ers defense had been gouged for 38.7 points and 404.3 yards per game and came up with zero takeaways over the final three weeks, Bosa struck a different tone when asked about Sorensen.

“I think he’s a good coach,” Bosa said. “But it’s not my decision.”

Indeed, that decision fell to coach Kyle Shanahan, who provided multiple votes of confidence for Sorensen throughout the season. But the defensive regression over the final three weeks left Shanahan ready to move on from Sorensen.

Now the the team is searching for its fourth defensive coordinator in as many years. Of the previous three, only DeMeco Ryans held the job for more than one season before he became the head coach of the Houston Texans. Steve Wilks was one and done after the 49ers made the Super Bowl last season. Unlike Wilks, Shanahan would like to retain Sorensen in a different role.

According to Shanahan, the choice to move on was as much about the options available as it was the defense’s 2024 performance. Unlike the past two years when the Niners had advanced deep into the playoffs and many of the top potential coordinators had already been hired elsewhere, they’re on equal footing with the opportunity to hire just about any of the available options this time around.

“It was a real tough decision,” Shanahan said. “I love Nick as a person and I love him as a coach… But just where we’re at, really as a team, where I think we need to go. There are a lot of big decisions ahead for us. And as hard as it is for me to come to this conclusion, I feel there are some options out there that can end up being a better option in the situation that we’re in for our team.”

That many candidates come with experience in the job is appealing to Shanahan. He turned to Sorensen because of his familiarity with the 4-3 scheme that Shanahan prefers, but also had never been a defensive coordinator before.

49ers coach Kyle Shanahan has interviewed former New York Jets coach Robert Saleh for the open defensive coordinator position — a position Saleh held before coaching the Jets. Darren Yamashita-Imagn Images

This year’s crop of candidates includes coaches who check both boxes. Headlining that list is Robert Saleh, who held the 49ers coordinator post from 2017-2020 before becoming the New York Jets head coach.

Saleh runs Shanahan’s preferred scheme, knows the key defensive leaders still on the roster and has proved adept at altering the defense when injuries or other factors have made it necessary. He interviewed for the job on Thursday and a reunion is possible, if not likely.

The complicating factor with Saleh is that he has head coaching interviews lined up with the Jacksonville Jaguars and Las Vegas Raiders. If he emerges as a top candidate for either job and those searches drag on, it will make it difficult for Shanahan and the Niners to wait as other coaches are hired.

Jeff Ulbrich, who replaced Saleh as the interim coach of the Jets and whom the Niners attempted to hire last year, could also be available depending on how things play out in New York. Ulbrich is a San Jose native who played linebacker for the 49ers for 10 years and also runs the scheme Shanahan prefers.

While Saleh and Ulbrich are considered top candidates, another name worth keeping a close eye on is former New Orleans Saints coach Dennis Allen. Allen has extensive experience, working eight years as a defensive coordinator in Denver and New Orleans, almost six seasons as a head coach with the Saints and Raiders and another year as the Saints senior defensive assistant.