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JC Tretter played left tackle for two seasons at Cornell. Earned a third-team All-American honor, first-team All-Ivy League. There was only one problem.

At 6-foot-4, 300 pounds, Tretter was undersized to be a full-time left tackle in the NFL.

“I’m obviously not Jonathan Ogden out there,” Tretter said, referencing the Hall of Fame left tackle who towered at 6-foot-9, 340 pounds for the Baltimore Ravens.

So the Green Bay Packers drafted Tretter in the fourth round of the 2013 draft to be their center, a position his body was built to play. Looked like he’d get his chance before the 2014 season. Former starter Evan Dietrich-Smith had signed a free-agent contract with Tampa Bay, leaving a job opening in the middle of the Packers' offensive line. There was only one problem.

A preseason knee injury placed Tretter on temporary injured reserve, unlocking the door for Corey Linsley to seize the starting job.

Fortune hasn’t been Tretter’s friend through his first three seasons in the NFL. Too small to start at his college position. Too unlucky to start at the position where he was drafted to play. His home on the island of misfit linemen could have been NFL purgatory, a dead end.

Instead, Tretter may be the Packers’ most valuable backup.

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Rare is the offensive lineman who can play all five positions. Most can play multiple. Some can play four. All five? That almost never happens.

“It’s good for us,” Packers offensive line coach James Campen said, “but it’s also great for him individually. It creates value in this league. It’s good for him.”

There wasn’t much need for depth on the Packers' offensive line last season. The same five starters were available each of the final 16 games, counting playoffs. Their consistent health was an anomaly.

This season, the Packers' offensive line returned to the NFL norm.

They’ve had their preferred five starters just seven times in 15 games this season. Some weeks, multiple starters have been on the sideline. The Arizona Cardinals took advantage of a depleted offensive line last week, sacking Packers quarterback Aaron Rodgers eight times and adding a ninth when backup Scott Tolzien was inserted.

Tretter is the lone Packers backup who can play all five positions. He has started three games at center this season while also lining up at right tackle, right guard and left guard. In Arizona, Tretter jumped from center to right tackle in the same game.

Campen, the former Packers center, said Tretter’s transition from interior to perimeter against the Cardinals was especially impressive.

“I couldn’t do that,” he said.

After left tackle David Bakhtiari missed the first start of his three-year career in Arizona, coach Mike McCarthy said he's offering him "every opportunity" to play Sunday against the Minnesota Vikings. Bakhtiari has not practiced this week, though McCarthy said he might get some reps Saturday or "for sure" early Sunday. His availability likely will be determined on game day.

Backup Don Barclay struggled mightily replacing Bakhtiari last week. He allowed four sacks and four quarterback hurries while being called for three penalties. Barclay started three games at right tackle earlier this season and played reasonably well. After a torn ACL forced him to miss all of last season, it’s reasonable to question whether Barclay still has the speed and athleticism to play on the left side, where he’s matched against the opponent’s top edge rusher.

Rodgers battered behind leaky offensive line

Campen wouldn’t answer that question when asked this week. With his history, Tretter could be a better option than Barclay at left tackle, though the transition from inside isn’t easy.

“That’s kind of a big jump,” Tretter said. “Obviously, as you move farther away from the ball, they move farther away from you.”

Though all five spots are part of an offensive line’s greater whole — they share a position coach and attend the same meetings — there is a vast gap between playing on the line’s interior and perimeter. The difference is not unlike playing receiver and tight end.

There is no greater jump for an offensive linemen than moving from center to left tackle.

A center lines up close enough to smell his opponent’s breath. While a guard may get one step before engaging his block, centers engage as soon as they snap the football.

Left tackles work in reverse. They kick four, five steps on the snap, mirroring the opponent like a point guard hounding the dribbler. Each kick step comes with purpose, designed to set up a block.

“That’s the main difference,” Tretter said, “is just the point of attack at a different spot. So you kind of have to sit back and mirror longer before initial contact, whereas at center you’re punching, then you’re mirroring after. At tackle, you’re mirroring and then punching.”

It’s not the only difference.

A center is charged with blocking defensive tackles who make 300 pounds look small. B.J. Raji. Terrance Knighton. Vince Wilfork. These are semi-trucks on an NFL field. They plug running lanes, consume blockers.

Left tackles are charged with blocking speedy pass rushers. Julius Peppers. J.J. Watt. Von Miller. These are sports cars on an NFL field. They wreak havoc on the quarterback.

“When you move out to tackle,” Tretter said, “you know you’ve got to get back, you’ve got to use more speed than punch, you have to make sure you’re not over your toes or anything like that. Whereas at center you can get that initial strike and work on that power first, and then sit back in the mirror. So, yeah, you definitely have to get used to the different type of guy going from the nose tackles to the speed-rush ends and rush linebackers.”

Tretter can do it because he’s smart, for one. Cornell is no place for scholastic chumps. Tretter has a firm grasp on the playbook. Rookies try to learn one assignment, Tretter said. Now, he understands the entire play.

Of course, his history is also beneficial. Too small to start at left tackle. Too unlucky to start at center. He might be the perfect backup to play both.

rwood@gannett.com and follow him on Twitter @ByRyanWood

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