Injuries of 49ers’ linemen have taken toll on offense

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After missing the third quarter of Sunday’s loss to the Raiders with an elbow injury, left guard Mike Iupati returned for the 49ers’ first series of the fourth quarter.

Then, 12 plays later, on the final offensive play of the drive, rookie center Marcus Martin exited with a knee injury and didn’t return.

Sound familiar? It should. After the offensive line was nearly impervious to injury the previous two seasons, the 49ers’ front five have been frequently felled in 2014.

Consider: The 49ers’ starting offensive linemen have combined to miss 16 starts and 1,186 snaps, which is 12 more starts and 679 more snaps than they missed in 2012 and ’13 combined.

In 2012, the 49ers were one of three teams to start the same offensive line every game and ranked third in the NFL in rushing yards per attempt (5.1). This season, only left tackle Joe Staley has started every game and their rushing yardage per attempt (4.04) ranks 22nd in the league and would be their lowest average since 2008 (4.03). In addition, the 49ers are on pace to allow 53 sacks, which would match the third most in franchise history.

The lack of continuity began well before the season. Right guard Alex Boone missed the offseason program in a contract holdout and didn’t report to the team until six days before the season opener.

The 49ers (7-6) have scored more than 17 points in one of their past seven games entering Sunday’s visit to Seattle. And running back Frank Gore, who is averaging a career-low 4.0 yards per carry, suggested the issues for the NFL’s 24th-ranked offense have started up front.

“We had a lot of injuries and that really hurt us this year,” Gore said to the Seattle media this week. “Each week there is someone missing. As an O-line, that can’t happen. Even in camp, our whole O-line hasn’t been together since we came in. As a lineman, you have to be together all the time. Practice, games, you can’t have one guy here this week, one guy missing here next week. That will hurt you and I think it has hurt us this year.”

The 49ers have started six different offensive line combinations this season and they could trot out a seventh against the Seahawks. If Martin is sidelined, Joe Looney would become the 49ers’ third starting center this season. Looney hasn’t started a game at center since he was a freshman at Wake Forest in 2008.

“Cohesiveness on the offensive line is very important,” offensive coordinator Greg Roman said. “And that’s something we’ve had to work through all season.”

The biggest issue has been at right tackle, where starter Anthony Davis has missed eight of 13 starts because of a hamstring pull, a sprained ankle and a concussion that has sidelined him for the past three games. In his place, Jonathan Martin has allowed a team-high six sacks, which is two more than any other 49ers lineman, according to Pro Football Focus. Despite starting just eight games, Martin is tied for the second-most quarterback hurries allowed (19) and has the lowest run-blocking grade (-5.3) among linemen.

In Sunday’s 24-13 loss at Oakland, the 49ers avoided running behind Martin: Each of Gore’s 12 carries went to the left of right guard Alex Boone.

The changes up front could help explain the 49ers’ lack of commitment to their once-vaunted ground game this season. They are on pace for 449 rushing attempts, which would be 49 fewer than they averaged from 2011 to ’13.

In past seasons, armed with an intact offensive line, they talked of imposing their will on opponents. This week, fullback Bruce Miller, while praising the offense’s versatility, acknowledged that defenses have dictated the terms.

“This year, we’ve definitely mixed it up a little bit,” Miller said. “Run game. Pass game. I think the best thing about our team is we can always change and evolve based on what we’re getting from a defense. Being able to make adjustments if they’re not going to allow us to run the football. And some teams are hell-bent on not letting you run the football.”

Eric Branch is a San Francisco Chronicle staff writer. E-mail: ebranch@sfchronicle.com Twitter: @Eric_Branch

O-line attrition

A look at the playing time of 49ers’ starting offensive line from 2012-14:

2012

LT Joe Staley: 986 of 1,032 snaps

LG Mike Iupati: 1,007 of 1,032

C Jonathan Goodwin: 1,007 of 1,032

RG Alex Boone: 1,031 of 1,032

RT Anthony Davis: 1,032 of 1,032

Missed starts: 0

Missed snaps: 97

Starting OL combinations: 1

2013

LT Joe Staley: 958 of 1,022

LG Mike Iupati: 715 of 1,022

C Jonathan Goodwin: 1,005 of 1,022

RG Alex Boone: 1,021 of 1,022

RT Anthony Davis: 1,001 of 1,022

Missed starts: 4

Missed snaps: 410

Starting OL combinations: 2

2014

LT Joe Staley: 884 of 888

LG Mike Iupati: 773 of 888

C Daniel Kilgore: 463 of 888

RG Alex Boone: 834 of 888

RT Anthony Davis: 300 of 888

Missed starts: 16

Missed snaps: 1,186

Starting OL combinations: 6