SEATTLE — Roughly nine minutes after the San Francisco 49ers pushed their lead to 20 points against the Seattle Seahawks on Thursday night, linebacker Fred Warner was hit with an uncomfortable case of déjà vu.

Seattle had just rattled off 14 straight points to cut San Francisco’s lead to six and was getting the ball back to open the fourth quarter.

For a Niners team that had blown double-digit fourth-quarter leads in losses to the Los Angeles Rams in Week 3 and the Arizona Cardinals on Sunday, it was hard not to let doubt creep in that it was going to be an NFC West trifecta.

“[A] thousand percent,” Warner said. “It sucks, but yes, it was something that felt familiar for sure. I’m like, ‘Hey, we can go one of two ways right here. We can stand tall in a hostile environment and get the game that we got to get or we can settle for exactly how we’ve been playing the last couple losses.”

This time, despite another rash of injuries and an unseen challenge replay that went against them, the Niners persevered to put away a 36-24 victory they badly needed.

During the week, the Niners proclaimed that Thursday’s contest was the closest thing to a must-win game as you can get in Week 6 of the NFL. A loss would have dropped them to 2-4 overall, 0-3 in the division and 0-4 in the conference. They still would have had 11 games to get back in the mix, but the climb would have been significantly more arduous.

The flip side, of course, was that a victory would propel the Niners to 3-3 and into a tie with Seattle at the top of the division with an early leg up against the Seahawks in a potential head-to-head tiebreaker.

“I’ve talked about some hard losses are harder than others and when you feel like you have those won, especially division games, that’s made us sick about it,” coach Kyle Shanahan said. “We’ve been talking about how these two losses are a reminder of how the NFL works, and I think we got a little spoiled in that way of just human nature of sometimes feeling too relaxed and you can never feel too relaxed.”

The mere idea of relaxation has been difficult for the Niners to grasp given all that they’ve dealt with on the injury front this season. Already without key players such as running back Christian McCaffrey (Achilles tendinitis), linebacker Dre Greenlaw (torn Achilles), defensive tackle Javon Hargrave (torn triceps) and safety Talanoa Hufanga (torn ligaments in his wrist), the Niners had more health woes Thursday night.

Cornerback Charvarius Ward went through a workout in hopes he could play through a knee bruise but was a pregame inactive. Rookie safety Malik Mustapha, starting in Hufanga’s place, came up with an early interception and then departed with what Shanahan called a low ankle sprain. Perhaps most critical, running back Jordan Mason injured his left shoulder in the first half, returned for one carry to open the third quarter and then was only available in an emergency situation the rest of the game. He finished with 73 yards on nine carries and caught another pass for 9 yards.

Shanahan said Mason will have further testing Friday to determine the severity of the injury.

“He thought he was going to be all right,” Shanahan said. “He went back in, and it just hurt him too much, so he went out. … We’ll find out more tomorrow.”

As if the injuries weren’t enough, they were also on the wrong end of a replay in which a camera angle eventually showed fans at home that Seahawks punt returner Dee Williams touched a ball that the Niners would eventually recover at Seattle’s 18. Shanahan said Brian Hampton, the 49ers vice president of football administration, alerted him to Williams touching the ball but when officials reviewed the play, they didn’t have an angle that provided clear evidence. That caused the call to stand and set off frustration on San Francisco’s sideline.

Mark Butterworth, the NFL’s vice president of instant replay, said after the game that they did not see the clear replay where the ball was touched until it was too late to overturn the play.

“With it being ‘Thursday Night Football,’ I thought for sure they’d have a bunch of camera angles,” Shanahan said. “We threw it thinking we’d get some better angles and then they just told me that he didn’t. And then about two minutes later, I heard all the guys in the box start freaking out saying they saw another angle and it was a fumble.”

As it turned out, that play didn’t matter as the 49ers closed the game with help from rookies working in replacement roles.

Cornerback Renardo Green, playing more snaps because Ward was out, came up with his first career interception midway through the fourth quarter to set up what turned out to be the winning 9-yard touchdown pass from quarterback Brock Purdy to tight end George Kittle. Rookie Isaac Guerendo, in for Mason, followed suit by ripping off a 76-yard run that all but sealed the win with 1:39 to go.

“We’ve been in a lot of big games, we’ve won a lot of big games,” Kittle said. “We’ve lost some big games, and so it’s just we are experienced in the moment. … When you have new guys out there and when they feel the confidence from all the guys who’ve been through there before, they feel like they fit right into it and they have that confidence as well. So, it’s just our team’s been in those situations, and we’ve done pretty good with those, especially here on the road in Seattle.”

While the 49ers get to enjoy Thursday’s win for a few extra days, they won’t take long to turn their attention to the next opponent on the schedule: the Kansas City Chiefs. The 49ers have never beaten Kansas City in a regular or postseason game since Shanahan took over in 2017, including, of course, a pair of Super Bowl losses.

Kansas City is 5-0 and will be even more rested than the Niners coming off their bye.

Suffice to say, nobody in San Francisco will call it a must-win. But there’s no denying that they want it.

“It’s not like we can get payback for losing in the Super Bowl,” Williams said. “That ship has sailed. … Obviously, we know what type of team that is, one of the best teams in the league, all-star quarterback, great coach. So, it’s going to be a tough game regardless. We’re going to take these three days off and regroup and come try to put a complete game together.”