GREEN BAY, Wis. -- First B.J. Raji, now JC Tretter. The Green Bay Packers have lost another starter in the preseason.

At least Tretter, the Packers' new starting center, won't miss the entire season like Raji.

But on Sunday, the Packers said Tretter will miss "multiple weeks" because of a knee injury he sustained in Friday's preseason game against the Oakland Raiders.

Packers coach Mike McCarthy said he did not know if the team would use the injured reserve/designated to return distinction, but according to a league source, Tretter's injury should not keep him out any longer than six weeks. A player on temporary IR has to miss eight weeks.

The source said Tretter has a slight fracture and did not sustain any ligament damage. He will not require surgery.

Raji, however, is not a candidate for that designation because he will undergo season-ending surgery.

Unlike Raji, who dropped out against the Raiders because of what turned out to be a torn right biceps, Tretter played through his knee injury and completed his playing time before the injury became apparent. Team doctors looked at Tretter's left knee between series in the first quarter, but Tretter returned to the game without missing any snaps. He played all 45 plays with the No. 1 offense against the Raiders.

JC Tretter should not miss more than six weeks with a slight knee fracture. Mike McGinnis/Getty Images

"Once the ones went out of the game, and once he settled down, that's when it started bothering him," McCarthy said Sunday. "It was bothering him at halftime, then when I got back in here after the game they ended up putting him on crutches."

It's a blow for the Packers, who spent the entire offseason preparing Tretter to start the season-opener at Seattle on Sept. 4. Tretter, a second-year pro, took every rep with the starters during OTAs, minicamp and in the first three preseason games. Their confidence in his ability to take over the position was part of the reason they allowed Evan Dietrich-Smith to leave in free agency and sign with Tampa Bay.

Now, the Packers have less than two weeks to prepare rookie fifth-round pick Corey Linsley of Ohio State to start at center against the Seahawks in perhaps the loudest stadium in the NFL. He also might have to do it without the benefit of any game reps with quarterback Aaron Rodgers, who typically does not play in the preseason finale. The center plays an important role in the Packers' no-huddle offense.

"The urgency level is just through the roof," Linsley said. "I've been working hard, but it's a different animal out there with the ones. I've got to fill in at the highest level. There's no room for nonsense anymore and ridiculous mistakes. [Missed assignments] are out of the question anymore, and the urgency level is hyped and I'm prepared for it."

It's the second straight year the Packers have lost a starting offensive lineman in August. Last summer, tackle Bryan Bulaga went down with a torn ACL that cost him the entire season.

If the Packers decide Linsley isn't ready for the job, they could move Sitton or T.J. Lang from guard and then use backup guard Lane Taylor as a starter.

"Happens every year, man," Packers guard Josh Sitton said. "We've dealt with it for a while around here and it's unfortunate, man, but it seems to happen every year. As bad as it sounds, I'm kind of used to it."