The 49ers’ struggles to finish: Is the offense or defense most to blame?

If the 49ers can’t scrap together a win over the Green Bay Packers at Lambeau Field this Sunday, their season will be on life support.

With a victory, the 49ers will be likely to make the playoffs. Their probability will shoot over 50 percent. But with a loss, the 49ers’ postseason chances will dwindle to about 15 percent.

No pressure.

Perhaps the 49ers should tell themselves those two words, since they’ve already wilted in so many pressurized situations this season.

Building three leads against NFC West opponents before blowing them in the fourth quarter has put the 49ers in an unusual position: Their efficiency profile far outpaces that of the typical 5-5 team.

And that’s precisely why the 49ers remain confident entering what might be the most brutally difficult stretch of this season. Their trip to chilly Green Bay to face the 7-3 Packers will be followed by a long return flight to the Bay Area, the Thanksgiving holiday and then a flight back across most of the country to Buffalo. The 8-2 Bills, one of the NFL’s elite teams, will host the 49ers in possibly snowy conditions there.

“We understand the task ahead,” 49ers edge rusher Nick Bosa said Wednesday. “We understand that we have to finish games. We’ve come very close in most of our losses. We’re probably three or four plays away from being an 8-2 team, which is easy to say, but it shows that if we can play better football throughout the game, we’re as good as anybody and we believe that. It’s just a matter of finishing in the fourth quarter.”

The problem is that the 49ers have been unable to do that so far. Let’s set special teams aside for a moment (that unit ranks No. 32 in expected points added) and look at some offensive and defensive trends.

49ers offense EPA ranks by quarter

While there certainly have been problems on the offensive side of the ball late in games — the 49ers’ failure to close out Seattle after quarterback Brock Purdy missed high to receiver Deebo Samuel on a basic third-down dig route is a prime example — the pronounced struggles over a larger sample, statistically, have come on the defensive side of the ball.

49ers defense EPA ranks by quarter

The loss to the Seahawks, which saw Seattle march down the field to score the winning touchdown with just seconds left, had a key extenuating circumstance. Bosa exited the game after halftime with another hip injury, draining the 49ers’ pass rush (they allowed 3.3 yards per play and no touchdowns with Bosa on the field and 6.1 yards and two touchdowns without Bosa).

But even entering that loss, the 49ers defense ranked No. 28 in fourth-quarter defensive EPA. Their relatively rare blitzes under first-year coordinator Nick Sorensen have been generally ineffective, as the team is also near the bottom of the efficiency charts when dialing up added pressure. That proved particularly problematic after Bosa exited Sunday’s game and took a huge chunk of the 49ers’ pass-rushing productivity with him.

“You have to be aware of why we’re losing games, especially when we’re up and defensively we can close it out, that’s what hurts me the most,” 49ers linebacker Fred Warner said Wednesday. “When you’re up in a game, if the other team doesn’t score, then you win. Defensively, we’ve got to be more on it.”

It’s worth noting that the 49ers’ run defense has eroded much more quickly than its pass defense this season. The run defense has been above average only in the first quarter — during which it ranks No. 3 EPA — before dropping to No. 22 and worse for the remainder of games. The pass defense has held steady in the top-10 until the fourth quarter, when it’s dropped to No. 28 EPA.

Perhaps that’s an indication that the 49ers, who entered this campaign having played 60 games over the past three seasons, are wearing down physically. Struggles against the run often indicate as much, and coach Kyle Shanahan did say Wednesday “it was pretty obvious how we tired out on that drive” to close the loss to Seattle.

And that’s where the special teams struggles must enter this discussion. Put simply, the 49ers defense may not have the freshness and depth necessary to contend with the enhanced chances that muffed punts, missed field goals, or missed touchback opportunities give opposing offenses. Better complementary football is a must.

Fresh off a game during which the 49ers offense managed only 17 points, complementary play was the spirit of tight end George Kittle’s thoughts. He focused on his own unit’s shortcomings.

“Our offense needs to perform better,” Kittle said. “Coach Shanahan gives us opportunities to go out there and close games, and we missed those opportunities. Whether it’s missed blocking on a run play by anybody, a bad cut, even Brock with a bad throw — we’ve just missed out on more of those opportunities than in the past.

“I think our defense is playing at a good enough level for us to win. I think we should be scoring 25-plus points a game to be able to win those games. To me, the pressure is on the offense.”

In actuality, the pressure is on all three phases. The 49ers need their various units to positively feed off of each other, as has been the case over so many recent seasons. That’s the only ticket to rediscovering sustainable success, especially with the road growing even more difficult ahead.

• Purdy, who is dealing with shoulder soreness, didn’t throw at practice. Shanahan said he wasn’t overly concerned with his QB’s availability for Sunday’s game. Purdy did footwork and hand-off drills.

• Bosa’s hip remains sore but he is feeling better than he did Sunday. Bosa said it’s too early to tell if he’ll be able to play in Green Bay. Kittle, though, reported that he’ll definitely be back in the lineup.

“I will be playing on Sunday,” Kittle said. “Very excited. Can’t pass up playing the Packers. I will be out there for sure.”

• Cornerback Charvarius Ward has rejoined the team following the death of his one-year-old daughter, Amani Joy. Ward had missed two games while on bereavement leave. The 49ers will assess how Ward is reintegrating at practice before determining how much he’ll play on Sunday.