Ask around the Vikings locker room why, a year ago, one of the NFL’s top scoring defenses fell flat in the season opener in San Francisco and you’ll receive different answers — but similar shoulder shrugs.

“Felt like we were kind of behind the 8-ball once it started as far as a physicality standpoint,” safety Harrison Smith said. “That’s something. Also it was West Coast, Monday night.”

“It was a lack of focus at times,” defensive tackle Tom Johnson said.

“We just had too many guys trying to make plays,” defensive end Brian Robison said.

One July evening before training camp, head coach Mike Zimmer outlined a dozen or so bullet points on his to-do list. One of them, better run defense, was likely sparked by the 49ers debacle — 13 missed tackles and 230 opponent rushing yards led to the Vikings’ loss.

And Sunday’s season opener in Tennessee has the makings for a nightmarish déjà vu.

Like San Francisco, the Titans have a new offensive staff and system. They feature a starry running back tandem in DeMarco Murray and Derrick Henry as well as an underrated running quarterback in Marcus Mariota, who led all passers with 7.4 yards per carry as a rookie.

The Vikings hope is there is no sequel to the film where 49ers tailback Carlos Hyde and quarterback Colin Kaepernick combined for 209 rushing yards.

“The two backs are excellent — Henry and DeMarco Murray,” Zimmer said. “They’re very strong at the tight end position. [Anthony] Fasano was in Dallas when I was there a long time ago. … The offense is different from it was a year ago. As far as personnel, it’s completely different.”

Alex Boone, who signed a four-year deal to join the Vikings this offseason, was on the delivering end of last year’s punishing loss. The former 49ers left guard got under Sharrif Floyd’s pads on two 8-yard gains in the opening possession. Now that he’s on the other side, Boone said his pregame message stays the same — just delivered to a different locker room.

“Punch them right in the mouth,” Boone said. “From the first play, and you let them know you’re going to be there.”

That physicality is how the Vikings feel they’ll be able to achieve an improved run defense. They’ve also increased practice repetitions this summer against myriad formations a year after the defense allowed 4.3 yards per carry to rank 21st in the league, Johnson said.

The preseason likely didn’t put Zimmer at ease after opposing backs averaged 5.6 yards per carry against Vikings starters in three exhibitions. That was often without three key players in linebacker Eric Kendricks, cornerback Xavier Rhodes and Floyd, who played sparingly or not at all.

All three are expected to start Sunday, putting the defense at full strength for a chance to prove this budding group is ready to join the likes of Denver and Seattle in the NFL’s upper echelon. The undisputed spearhead is the Vikings’ pass rush, one that mixes waves of linemen with varied fronts designed to confuse blockers and quarterbacks.

To unleash those QB hunters, they’ll need to prevent third-and-short situations often created by a successful running game.

“If we can do that,” Robison said, “we’ve shown we can be a top-five defense.”

The Vikings also plan to have a few new tricks up their sleeve since last season. They’ve installed at least one new defensive package, nicknamed “Penny,” which is essentially the base defense with an extra cornerback, Captain Munnerlyn, instead of a strong safety.

They’ll also have Kendricks, the team’s first league-leading rookie tackler since 1961, after he didn’t start in the middle until Week 4 last season. And run-stopping specialist Shamar Stephen, a versatile defensive tackle, is healthy after last year’s season-ending turf toe injury that forced the Vikings to lean on Johnson, who is better rushing the quarterback.

All hands should be on deck to see if a young, and potentially star-studded, defense has grown.

“I don’t know,” Zimmer said. “We’ll find out.”