In the first half of last season’s NFC Championship Game, the Lions scored 24 points, converted 5 of 7 third downs, scored touchdowns on two of their three red-zone trips and didn’t commit a turnover.

Eleven months later, as Kyle Shanahan reviewed those two quarters this past week, the San Francisco 49ers’ head coach felt like he was rewatching a nightmare that would take a dramatic turn: The 49ers, of course, beat Detroit 34-31 after facing a 17-point halftime deficit.

“You watch the first half and you get frustrated,” Shanahan said. “But you know how it’s going to end, so you don’t get too frustrated.”

This season, those same defensive issues from early in the NFC title game — an inability to stop opponents due to struggles on third down, the red zone and in creating takeaways — have plagued the 49ers throughout their first 15 games. And it’s a big reason this nightmare won’t have a happy ending.

The 49ers will drag a 6-9 record into Monday night’s rematch against the Lions (13-2) at Levi’s Stadium and their defensive shortcomings have raised questions about just how frustrated Shanahan might be with the man in charge, rookie coordinator Nick Sorensen.

Last season, the 49ers’ defensive performance in their two playoff wins — escapes against the Packers and Lions in which they surrendered 318 rushing yards — was a reason first-year coordinator Steve Wilks was dismissed despite overseeing a unit that ranked third in points and eighth in yards allowed per game.

In 2024, the 49ers entered Week 17 ranked second in yards allowed per game, but that’s about it for statistical bright spots. They are 19th in points allowed, which would be their lowest ranking since 2018, largely because they are tied for 17th in the NFL in takeaways, are 24th in third-down defense and 29th in red-zone touchdown percentage.

Asked about Sorensen’s performance, Shanahan referenced some of the 49ers’ injury issues in his assessment.

“I think Nick has done a good job as a coach,” Shanahan said. “No one’s happy with our results by any means. I think he’s been thrown in a number of situations that I think will make him better going forward. I’ve watched him adjust to a bunch of different situations.

“He’s kind of been in a couple situations in a couple games when there wasn’t much of an answer — and he still had to try to find one. So I think this year was good for him. I know he is going to be a good coach and better for it going forward.”

But will Sorensen continue to grow with the 49ers? Even the circumstances of his promotion in February raised questions about his long-term job security. The 49ers elevated Sorensen only after looking into Bill Belichick, Chiefs defensive coordinator Steve Spagnuolo and Jets defensive coordinator Jeff Ulbrich, who is now New York’s interim head coach and figures to be available in the offseason.

At the time the 49ers tabbed Sorensen, they also added former Rams defensive coordinator and Chargers head coach Brandon Staley as their assistant head coach after Staley had interviewed for the position.

The subsequent performance of Sorensen’s unit hasn’t screamed that he’s guaranteed a second season as coordinator. Still, there are compelling reasons to believe he could return in 2025.

Unlike Wilks, whose background in a different defensive scheme Shanahan cited when he was fired, Sorensen has an extensive background in the 49ers’ 4-3 system. That was the case with Shanahan’s first defensive coordinator, Robert Saleh. And Shanahan retained Saleh, who also had no prior coordinator experience, after the 49ers ranked 25th and 28th, respectively, in points allowed in his first two seasons (2017-18).

Saleh was dealing with a roster that was being rebuilt. And Sorensen has dealt with a roster that has been ravaged by injuries. Last season, the middle of the 49ers defense included two stalwart defensive tackles, since-released Arik Armstead and Javon Hargrave, along with linebackers Fred Warner and Dre Greenlaw. This season, Greenlaw and Hargrave combined to finish two games and Warner has played the past 11 games with a broken bone in his ankle.

The attrition is a reason why the 49ers have the NFL’s 15th-ranked run defense and have allowed four 100-yard rushers in their past five games after allowing none in their previous 55.

Sorensen cited injuries, and preparing players to assume different roles, when asked what he has learned this season.

“I’ve got to keep learning and keep growing and being on top of situations where you might, always picturing personnel stuff because we had so much of it this year,” Sorensen said. “So it kind of happened a few times early in the year and it really helped me moving forward.”

Sorensen and 49ers will face their biggest test Monday against Detroit, which leads the league in points per game and ranks second in yards under the supervision of coordinator Ben Johnson, who will be a hot head-coaching candidate in the offseason.

If Shanahan hasn’t already determined Sorensen’s fate, a strong performance against a team determined to finish the job in a rematch could bolster his case.

“Ben Johnson’s a really good coordinator — he knows exactly what he’s doing,” Sorensen said.  “He knows how to game plan for you and then he’s got his wrinkles … So it’s a tough task because they’ve got a great scheme and great coaching all the way around.”

Reach Eric Branch: [email protected]; X: @Eric_Branch