SANTA CLARA, Calif. -- In the midst of a stunning 9-1 start, a dominant San Francisco 49ers defense drew comparisons to some of the best units the NFL has seen.

The Niners were mentioned alongside the Tampa Bay Buccaneers and Baltimore Ravens of the early 2000s as well as the Seattle Seahawks' "Legion of Boom" era. In fact, the Niners were keeping a historic pace with those clubs in categories such as sacks and pass defense.

Despite the start, it was fair to wonder if the 49ers could continue their dominance when the schedule toughened and injuries began to strike. As it turned out, they couldn't.

Over the season's final six weeks, the 49ers' defense proved vulnerable and the group that led that 9-1 start suddenly required more help from the offense to sew up the NFC's No. 1 seed and the NFC West Division crown.

Now, as they head into Saturday's NFC divisional playoff game against the Minnesota Vikings (4:35 p.m. ET, NBC), the question becomes: What happened to the Niners' defense and can it return to dominance?

"From a statistical standpoint, it was almost historic over those first eight games," defensive coordinator Robert Saleh said. "I almost feel like we kind of spoiled people. We've still been playing good defense and doing all those things. It's something that we've got to just continue to keep grinding and getting better with the different guys that are coming in and out of the lineup."

Through their first 10 games, the 49ers were first in the league in QBR allowed, passing yards allowed, sacks and sack rate; second in yards allowed per game, yards allowed per play and points allowed; and third in takeaways.

A variety of factors pushed the defense to a new level this season. The 49ers got offseason additions at edge rusher in Nick Bosa and Dee Ford as well as at linebacker in Kwon Alexander. Richard Sherman returned close to his previous form as one of the game's best cornerbacks. The Niners also continued to see the ascent of linemen DeForest Buckner and Arik Armstead as well as linebacker Fred Warner.

Along the way, as the offense struggled with injuries, the defense mostly remained intact. But in Week 9, the injury bug hit and didn't stop until it had ravaged all levels of San Francisco's defense.

Alexander, who had established himself as the defense's emotional heartbeat, suffered a torn left pectoral in an Oct. 31 game against the Arizona Cardinals, landing on injured reserve. The following week against Seattle, reserve lineman Ronald Blair III tore an ACL, ending his season. A week after that, Ford strained a hamstring against the Cardinals, an injury that would limit him to a brief four-snap cameo against the New Orleans Saints, his final appearance of the regular season. End Damontre' Moore, signed to fill in for Blair, broke a forearm the next week against the Green Bay Packers, and strong safety Jaquiski Tartt broke some ribs against the Baltimore Ravens on Dec. 1.

That Saints game on Dec. 8 also resulted in the loss of run-stuffing defensive tackle D.J. Jones to a season-ending ankle sprain and short-term issues for Sherman and nickel corner K'Waun Williams in addition to Ford.

The lack of depth up front and the missing starters in the back seven triggered a trickle-down effect that forced the Niners' starting front to play more than it had early in the season. The gas tank ran perilously low for the starters. In the first 10 games, Armstead, Buckner and Bosa combined to average 124.5 snaps per game. In the final six games, that number jumped to 162.3, or almost 13 more snaps per game per player.

It's no coincidence that a trio that combined for 20 sacks in the first 10 games had 6.5 over the final six.

"The less depth you have, the more reps people take, and the more reps people take, they can't go as hard for every time they're in," coach Kyle Shanahan said. "I think guys do wear down a little bit with that. It's something you fully expect this time of year, as the year goes, but it does add up."

Despite that attrition, the 49ers managed to win four of their last six while taking on top quarterbacks and postseason contenders such as Drew Brees' Saints, Lamar Jackson's Ravens, Aaron Rodgers's Packers and Russell Wilson's Seahawks. Perhaps most important, they wrapped up a first-round bye that allowed them to rest and heal their many wounds.

Tartt's absence has affected communication on the back end with Marcell Harris, in his second NFL season, taking over. Alexander's stickiness in coverage and infectious energy had Bosa recently calling him "probably the MVP of our team," but he was noticeably missing in a lackluster loss to Atlanta.

There's now a chance the Niners will go into Saturday's game against the Vikings with Ford, Tartt and Alexander available in at least some capacity. Tartt and Alexander appear to be good to go, and though Ford is listed as questionable, there’s a chance he’ll at least be available on a limited basis. If that happens, the defense would be as close to healthy as it has been since the 9-1 start.

Ford's possible return might have the biggest impact, even if he plays fewer snaps than Tartt and Alexander. According to ESPN Stats & Information research, the Niners had 24 sacks on 164 snaps with Bosa and Ford on the field together. They had 24 sacks on 801 snaps on all other plays.

And while the statistics haven't been as bad overall as some might think during the final six games, there's no doubt a return to the quarterback-harassing, turnover-creating group that spurred the early surge would go a long way in helping the 49ers turn their Super Bowl dreams into reality.

"I don't think we're really looking in the past," Bosa said. "Everybody loves to talk about how we were historic in the beginning and the numbers aren't there anymore, but we are the No. 1 seed, so obviously we're doing something right. And obviously getting Dee and 'Quaski and Kwon back is going to help a lot."