San Francisco 49ers' Eric Reid (35) and Donte Whitner (31) break up a pass intended for Seattle Seahawks' Jermaine Kearse during the second half of the NFL football NFC Championship game Sunday, Jan. 19, 2014, in Seattle. (Associated Press - Ap/The Minnesota Star Tribune)

When the Vikings offense comes out for its first possession Monday night, Teddy Bridgewater and Co. will look across the line of scrimmage at a 49ers defense that is drastically different from their top-10 unit of a year ago.

49ers inside linebackers Patrick Willis and Chris Borland both unexpectedly retired in the offseason. That standout defensive end Justin Smith hung them up was less of a surprise, but it still stung. Outside linebacker Aldon Smith was booted off the team after his latest brush with the law. And both of San Francisco's starting corners were allowed to leave in free agency.

The 49ers could have as many as seven new starters on defense this season. But they have invested heavily on that side of the ball in recent years, spending first-round picks on safeties Eric Reid and Jimmy Ward and defensive end Erik Armstead. They have also used second-day picks on front-seven defenders Tank Carradine, Corey Lemonier and Eli Harold.

"I know people are talking about the players they've lost," said offensive coordinator Norv Turner. "But in terms of looking at them they still have a group of guys that play at a high level and play extremely hard."

And the fact that this new group of guys has yet to play a regular-season game together presents a challenge for the Vikings coaches and players as they prepare for Monday night. However, it does help that the Vikings have had weeks to get ready for their season opener against the 49ers.

So considering that the 49ers have all these new starters — and, oh, a new defensive coordinator in Eric Mangini — have the Vikings been spending their time looking at tape from last year or the 49ers during this preseason?

"Both and more," coach Mike Zimmer said. "We look at Mangini, who was a coordinator before, when he was a head coach, what they did defensively. We look at the preseason. We go back and look at last year, especially their personnel, the players like [linebackers] NaVorro Bowman and Ahmad Brooks and those guys, Reid the safety. We do our due diligence."

The 49ers allowed 60 points in four preseason games, picked off five passes and forced six fumbles. Their opponents averaged 4.5 yards per play. But the 49ers allowed them to convert 41.5 percent of their third-down plays.

The starters were only on the field for a fraction of that, but that small preseason sample size is the best the Vikings can do when it comes to getting a look at how all the new pieces in this 49ers defense might fit together.

"You know, it is tough. They have a new coaching staff, seven different starters on defense this year," Bridgewater said. "[You] just try to judge them off of the preseason, but some teams don't show everything in the preseason. We know that there are going to be some surprises come Monday night. They're going to do some things that we didn't see them do in the preseason and that's where we just make adjustments on the run."