SANTA CLARA, Calif. - The San Francisco 49ers did everything the could this offseason to be more multiple on offense with their personnel groupings to prevent teams from stacking the box.

Sunday’s opponent, the Chicago Bears, might be their polar opposite.

In Sunday’s loss to the Buffalo Bills, the Bears only used two personnel groupings*, 11 and 12, according to Pro Football Focus. In contrast, the 49ers used five in Week 1 against the Cowboys. (*explanation below.)

The Bears and the 49ers define offensive versatility in different ways. In recent seasons, San Francisco has mixed and matched groupings according to situations and game plans, while Chicago likes to keep mostly the same group on the field.

Offenses’ plays can be predictable based on personnel, but the Bears combat that using their core of talented playmakers in receivers Brandon Marshall and Alshon Jeffery complimenting running back Matt Forte, who finished second in the NFL in rushing last year with 1,339 yards.

Simply put: there are fewer clues as to what the Bears might be doing before the snap by keeping the same players on the field.

”We can’t anticipate certain plays,” linebacker Michael Wilhoite said. “As a defense we’ll look for other hints, or other keys to look at. But for the most part it does make it more difficult.”

With the 49ers’ defense in relative flux with Aldon Smith and NaVorro Bowman out, on top of potentially missing starting corner Tramaine Brock (toe), the defense will have a lot on its plate. Chris Culliver (concussion) participated in Thursday’s practice and looks to be on track to play Sunday, but must first pass the league's mandatory concussion protocol.

The key to Chicago’s offense is Forte, who ranks third in team history with 349 receptions, including 74 last year.

”One of the reasons they don’t maybe have as many packages as some other teams is because (Forte) is a viable receiver out of the backfield,” 49ers defensive coordinator Vic Fangio said. “So like last week you saw Dallas go to four wide receivers and one tight end. Well if Chicago wants to something along those lines they just do it with 22 (personnel) in the game. So his versatility allows them to do more out of one personnel group than other teams. And that makes it harder on defense.”

Neither Marshall (ankle) or Jeffery (hamstring) have participated in practice this week after sustaining injuries in their season opening loss at home to the Bills, which could mean Forte plays an even bigger role Sunday night. He totaled 25 touches for 169 yards against Buffalo.

”He’s one of the best all-purpose in the National Football League,” linebacker Patrick Willis said. “Most definitely we have to make sure we know where he is at all times, whether it’s him coming out of the backfield on check downs, or them making routes for him, whatever it may be.”

San Francisco allowed 118 yards on 22 carries to Dallas’ DeMarco Murray in Week 1, marking the first time they allowed a 100-yard rusher in the regular season since 2012, when Marshawn Lynch had 111 in a December blowout.

"(The run defense) wasn’t good last week," Fangio said. "We did not have a good game up front versus the run and rushing the passer, for the most part...We’re going to need to play the run a whole lot better in this game than we did the last game."

The Cowboys’ four turnovers were the story of the opener, but Dallas out gained the 49ers 382 to 319 leaving San Francisco’s defense with a bad feeling heading into Week 2.

”That was the first time all of us being out there live, playing an all-out game,” said Willis. “In the preseason…you have some injuries here and there and you don’t play as much. (Dallas) was kind of like a warmup game in sense of it being our first real live action.” Rookie cornerback Dontae Johnson stepped in after Brock and Culliver left the game in the first quarter. He made an impressive debut with two pass break ups while being matched up with talented wideout Dez Bryant.

Both Marshall and Jeffery, who are day-to-day, possess similar size to Bryant. Johnson, 6’2”, will use his height to his advantage should he be needed to play against Chicago. He expects to be targeted by Jay Cutler frequently.

”That’s one of the things that comes with being a rookie. You got to be able to step up to the challenge, step up to the plate, and make plays when presented to you,” Johnson said.

*Editor’s note- A quick lesson on identifying personnel groupings: the first number is the number of runnings backs, while the second is tight ends. Because five linemen and a quarterback are on the field at all times, the number of running backs, tight ends and receivers can be no more than five. The number of receivers on the field is deduced by subtracting the grouping number from five. “11” is one back, one tight end and three wideouts - equalling five. “12” is one back, two tight ends, two wideouts. Not that complicated, right?

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