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The worst-case scenario unfolded in Green Bay Packers coach Mike McCarthy's mind.

Take a backup offensive lineman who had played only 50 snaps this season. Match him against a three-time Pro Bowl defensive end with almost 90 career sacks. Back the offense into the shadow of its goal post, down a touchdown with less than 2 minutes left and no timeouts.

Here's the recipe for disaster.

McCarthy knew it as the Packers started their final drive against the Buffalo Bills on Sunday. Before defensive end Mario Williams' game-clinching strip-sack, McCarthy considered adding an extra blocker to help right tackle JC Tretter. Instead, Tretter was left to block Williams one-on-one. He was beat off the snap.

"That last call is my fault," McCarthy said after the game. "I had another play in mind, and I should have went with it. The result would have been different. In that particular spot, I should have helped him on that play. That last fumble was my fault based on the protection call."

McCarthy's disappointment was with how Williams' sack impacted the game, but also how it overshadowed Tretter's otherwise solid outing.

There's a reason McCarthy didn't double-team Williams on Green Bay's final offensive play. Tretter hadn't needed help since entering early in the fourth quarter, relieving starter Bryan Bulaga, who suffered a concussion.

It was a small sample size, but Tretter allowed no pressures in his other eight snaps in pass protection. Green Bay mostly went with quick, short passes with Tretter at right tackle, but not exclusively. Tretter stood up against Williams' bull rush multiple times, including a third-and-8 that ended with Aaron Rodgers overthrowing a well-covered Randall Cobb in the end zone.

Then, the strip-sack happened.

With the Packers 90 yards from a touchdown, Williams switched things up. He used a speed rush around the edge instead of a bull rush through Tretter's chest, taking advantage of his athleticism.

"Obviously I'd like to have that last one back," Tretter said Monday. "… He just speed-rushed outside, and I was a little late getting out there. I just have to get out there a little quicker than I did."

Tretter has no time to dwell on that last play. With Bulaga undergoing concussion protocol, Tretter is on call this week. If Bulaga is unable to play Sunday in Tampa Bay, Tretter will get his first career start.

These relief appearances and spot starts are the reality for a player slotted as backup at all five positions on the offensive line. Each practice is an exercise in multi-tasking. Tretter gets regular reps at each offensive line position. He's expected to understand them as if he was the starter.

This week, he could be.

"I'll probably take more right tackle reps than I have been, just because it's the position of most need right now," Tretter said. "But I've been preparing for all the positions every week for the past several weeks now, anyways. So I don't think anything is going to change preparation-wise. I'll probably just get a few more reps there, but that's it."

Offensive coordinator Tom Clements isn't worried about Tretter recovering from the sack. The second-year offensive lineman has plenty of experience handling adversity.

Tretter played left tackle in college at Cornell, but he wasn't supposed to line up on the edge with Green Bay. Throughout the offseason, he was groomed to be the starting center. Two weeks before the season opener, Tretter injured his knee in the third preseason game.

Tretter was placed on short-term injured reserve, and rookie Corey Linsley flourished in his absence. By the time Tretter returned during the bye week in early November, Linsley had locked up the starting job.

Since then, playing time has been limited. Before Sunday, his only snaps came in garbage time, after the Packers had blown teams away in the first half. Finally, against the Bills, Tretter got his first meaningful snaps of the season.

"I'm sure he was happy to get in," Clements said. "Not because of the circumstances. You never want to see a guy get hurt in order to play, but it was a close game in a hostile environment. Overall, he did well."

Clements said Tretter's athleticism and understanding of the game allow him to balance playing on the interior offensive line and at tackle. His versatility has given the roster flexibility. With Don Barclay on injured reserve and Derrek Sherrod cut, Green Bay has gone most of its season with only two full-time tackles.

The Packers have had uncommon health along the offensive line, with the same five starters playing 11 straight games. Tretter's presence gives Green Bay a safety net when there is an injury at tackle. He also was used as a guard against Chicago, and an extra blocker at tight end against Atlanta.

"It's very helpful," Clements said. "If a guy can play multiple positions, then he really counts maybe as more than one guy. There are some players — not speaking specifically about our line — but some players can only play one position. If you can play multiple positions, you have value."

There's no guarantee Tretter will start this week. His snaps depend on Bulaga's availability.

He didn't make an excuse for his strip-sack against Williams. Tretter was close to being a starter this season. Even as a backup, that mentality hasn't left him.

"That's not unfair. That's my job," Tretter said of blocking Williams one-on-one. "That's why I'm on this team — to come in and play that position, no matter how many reps I've gotten. That's the position. That's what I'm supposed to do. So it's not unfair."

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

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