GREEN BAY, Wis. – Nearly all of his 49ers teammates had already zipped up their bags and headed for the buses, but Christian McCaffrey remained in the cramped locker room at Lambeau Field. He sat seething, stone-faced, staring into space after the Green Bay Packers defeated the 49ers 38-10.

Figuratively, McCaffrey might have been looking ahead to the long run of these 49ers’ ongoing torture season. The team is now 5-6 and the road won’t get any easier next week. The 49ers will travel across the country to take on one of the league’s best teams, the 9-2 Buffalo Bills.

Another performance that in any way resembles this one will almost certainly sink the 49ers, whose litany of big mistakes in Green Bay was too long to count on two hands.

“I just need to be better,” McCaffrey muttered.

Kyle Shanahan, head coach of the 49ers, said: “We were all embarrassed.”

In terms of points differential, this was the third-worst loss of Shanahan’s tenure, trailing only a 40-10 blowout to the Dallas Cowboys in 2017 and a 39-10 shellacking of the Los Angeles Rams in 2018.

Those were 49ers teams short on talent. It’s a safe bet to say this week’s 49ers — who were missing starting quarterback Brock Purdy, top defensive back Nick Bosa and All-Pro offensive tackle Trent Williams, among others — are in the same boat.

But the Packers, who fell to the 49ers in the playoffs last season, were not sympathetic.

“It’s the NFL,” Green Bay defensive back Keisean Nixon said. “It’s not an excuse. We didn’t have a quarterback. We won three games, so we don’t want to hear that. We came to play. They should have come to play.”

Instead of doing that, the 49ers defense opened up in the worst way imaginable. The Packers, on their way to building a 17-0 lead, rushed for 125 yards in the first half alone. They converted six of their first seven third downs and didn’t seem bothered at all by the extra blitz pressure that first-year 49ers defensive coordinator Nick Sorensen was calling on.

“The run defense was really disappointing,” Shanahan said. “We came out of the holes too often. Way too many missed tackles. That was one of the worst halves I’ve been a part of.”

The 49ers missed a whopping 10 tackles in the first quarter – the most by any NFL team in a single frame this season – and ultimately missed more than 20 tackles in the game. The Packers ended up scoring nearly 40 points despite only allowing 163 yards from their quarterback, Jordan Love.

In that way, this seemed like at least a little bit of payback for one of the 49ers’ highlights under Shanahan, a 37-20 thrashing of Green Bay in the 2019 NFC Championship Game. The 49ers lit up the scoreboard that day as they only eight passes attempted.

“Poor technique, poor execution across the board,” 49ers linebacker Fred Warner said. “We knew the challenge their running backs gave us going into the game, but we just didn’t execute.”

The 49ers were not a good defense entering Sunday, ranking No. 21 in expected points added. But this epically poor performance, even though the loss of defensive tackle Jordan Elliott to a concussion undoubtedly hurt matters, highlighted just how dangerously dependent the 49ers are on Bosa. The entire defense had crumbled the week before against the Seattle Seahawks after Bosa left with an oblique injury and looked downright incompetent for far too long in Green Bay.

Conversely, Sunday’s inept offensive performance makes Purdy’s value to the 49ers all the more apparent. The young quarterback, who has been out with a shoulder injury (Shanahan had no update on his possible availability for next week), has provided a stabilizing force that the 49ers sorely missed when they took this drubbing.

The 49ers fumbled five times and lost two to the Packers. Backup quarterback Brandon Allen also threw an interception when his pass ended up in the hands of receiver Deebo Samuel and off Green Bay safety Xavier McKinney in the second half. Allen had avoided an interception in the first half on a terrible pass straight into coverage, but couldn’t avoid a giveaway later when Packers edge rusher Lukas Van Ness sacked him and forced a fumble.

The 49ers also committed nine penalties, many of which came at particularly damaging moments. Notably, the defense drew flags as 12 men were on the field on consecutive snaps, and the offense seemed to continually struggle to contain the noisy Lambeau environment at the line of scrimmage.

“This league is built on defensive lines flying up the field and getting off the ball,” 49ers receiver Chris Conley said. “We have to be more disciplined when we use our cadence. We have to keep repeating it. There’s no way around it. You have to lock up. You can’t make those mistakes.’

The most damaging mistake was Samuel’s drop, which culminated in the pick that returned McKinney 48 yards. The 49ers were trailing just 17-7, but that play allowed the Packers to take a 24-7 lead.

“It looked like a great throw,” Shanahan said. ‘It went out of Deebo’s hands. That was huge because it looked like we were trying to get back into the game.”

Instead, the 49ers never got close. Their only consolation in leaving Green Bay, as Purdy and Bosa’s return schedules remain unclear, was that the rest of the NFC West remains mired in mediocrity. Seattle defeated the Arizona Cardinals to create a 6-5 team tie at the top of the division. Remarkably, the 49ers are just one game behind the leading pack, so they are far from mathematically eliminated.

But they were definitely injured leaving Lambeau.

“That’s about as bad as it can get,” Warner said. “It’s probably the worst thing I’ve been a part of. It’s embarrassing. You have to take it on the chin. Take it like a man and move on.”

That’s what McCaffrey was processing as he sat in the corner of the locker room, seething.

“There’s always a fight,” he said. “It’s one game at a time. Every day you have to wake up, look yourself in the mirror and get better, whether you win or lose. Speaking for me personally, that’s what I’m going to do.”