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This is the fourth in a series looking at positional needs for the Detroit Lions in free agency. Teams can begin negotiating with free agents on March 12. The signing period opens March 14 at 4 p.m.

Allen Park — During his two years on the job, Detroit Lions general manager Bob Quinn has poured more attention and resources into his offensive line than any other position group. Unfortunately, he has yet to see a return on that investment.

Last season’s starting lineup, which looked great on paper, was ultimately derailed by injuries.

Left tackle Taylor Decker, the team’s 2016 first-round pick, missed the first half of the season after an offseason shoulder injury required surgery. Center Travis Swanson battled through early knee and ankle injuries before missing the final three games with a concussion. And free-agent additions Rick Wagner and T.J. Lang each had issues that lingered throughout the year, especially Lang, with nagging foot and back injuries.

In total, the team used 10 different starting lineup combinations, which no doubt contributed to the career-high 47 sacks taken by quarterback Matthew Stafford and the league’s worst ground game.

But if the individual pieces have better fortune staying on the field in 2018, there remains reason for optimism. When healthy, Decker has looked the part of a premier blindside blocker. Lang, despite his injuries, was solid his first year in Detroit, making the Pro Bowl as an alternate. And Wagner is a quality right tackle, particularly in pass protection. Plus, Graham Glasgow, the only lineman to play every snap in 2017, showed significant improvement in his second season.

In all likelihood, Glasgow will be moving to center next season. Swanson is a free agent, and given durability concerns and his disappointing performance prior to landing on injured reserve, the team is expected to move on.

Assuming the Lions don’t have Joe Dahl penciled into the job, that leaves an opening in the starting lineup for Quinn to complete his overhaul of the unit. And if he wants to plug the hole at left guard in free agency, there’s some options on the market.

Quinn has implied the Lions won’t make as big of a splash in free agency as they have the past two years, signing wide receiver Marvin Jones in 2016 and Lang and Wagner last year. But if the GM reverses course on his thinking, Carolina’s Andrew Norwell is the best option available.

A first-team All-Pro, Decker’s former teammate at Ohio State would be a stellar addition. That said, a number of reports have Norwell heading to the Giants, where former Panthers GM Dave Gettleman is now at the helm.

Assuming Norwell is off the table, four-time Pro Bowler Josh Sitton is available after the Bears declined the option on his contract. At 31 years old, he’s still an excellent player. Lang, who played with Sitton in Green Bay, took to Twitter to make a recruiting pitch.

Another possibility would be leaving Glasgow at guard and bringing in Giants center Weston Richburg. A proven pass protector, the big concern would be his health. Like Swanson, Richburg’s 2017 campaign was ended by a lingering, long-term concussion.

Finding the final piece to the puzzle, combined with an overhaul of the offensive line coaching staff, could be the key to unlocking the offense’s potential. If Stafford is given more time to process in the pocket, and complemented by even an average run game, the Lions could truly be one of the league’s more potent attacks.

Lions OL under contract: Taylor Decker, Rick Wagner, T.J. Lang, Graham Glasgow, Joe Dahl, Corey Robinson, Brian Mihalik, Emmett Cleary, Dan Skipper, Leo Koloamatangi

Lions free agents: Travis Swanson, Greg Robinson, Don Barclay

Top free agents: Nate Solder, Justin Pugh, Andrew Norwell, Weston Richburg, Chris Hubbard, Josh Sitton, and Josh Kline.

MORE COVERAGE

Lions need LB depth as decision on Whitehead looms

Free agency offers options for Lions’ ailing run game

Lions need pass rush help but free agency unlikely cure

jdrogers@detroitnews.com

twitter.com/Justin_Rogers

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