GREEN BAY, Wis. -- The Green Bay Packers spent nearly five months preparing JC Tretter to start at center when the regular-season opens on Sept. 4 at the Seattle Seahawks.

Now they have less than two weeks to prime rookie Corey Linsley for the task.

The news that Tretter sustained a knee injury in Friday's preseason game against the Oakland Raiders and will miss up to six weeks struck a bigger blow than the loss of nose tackle B.J. Raji, who sustained a season-ending torn biceps in the game.

Raji is the bigger, more recognizable name, but Tretter is the greater loss.

Tretter is the reason the Packers opted not to re-sign last year's starting center Evan Dietrich-Smith, who went to Tampa Bay in free agency.

The Packers felt so confident in the second-year pro even though he had never played center that they devoted their entire offseason to getting him ready to help Aaron Rodgers run the no-huddle without so much as looking at another option in practice. Rodgers was going to have his fourth different starting center in as many years anyway. Now it will be a center he might not take a game snap with before the opener given that Rodgers does not typically play in the final exhibition game.

"It can be done," said backup quarterback Matt Flynn, who has worked extensively with Linsley this summer. "It's the 10 other guys around him that are key factors in not letting there be a setback. It's a blow for us, spending that much time with a guy getting him ready, getting him prepared. But the thing about training camp is everyone's getting about the same amount of reps. We've also been getting Corey ready. We hope that there's not going to be a drop off there. And he couldn't have any better people surrounding him, whether it's the two guards [Josh Sitton and T.J. Lang] or Aaron behind him, with the mental part of it."

It will be a crash course. The Packers have just two more practices this week before the preseason finale against Kansas City on Thursday and then have a short week to prepare for the Thursday night opener against the defending champs.

"This will be my millionth center, so it's not anything new to us," said Sitton, who also took some snaps at center on Sunday. "I wouldn't say it's difficult; it's just a process. I told JC this morning, 'Right when I got real comfortable with you, now you're gone.' It sucks. It's unfortunate, but we're used to working with a bunch of guys, so it's something you've got to roll with."

Before Sunday's practice, Linsley had not taken a single rep with the No. 1 offensive line since the time he was drafted in the fifth round out of Ohio State in May.

The physical aspect of the game does not look like a problem for him. For example, Linsley's record in the one-on-one pass-blocking drill in training camp is a respectable 8-2. However, offensive line coach James Campen has had to harp on Linsley about mental mistakes and missed assignments in both practice and preseason games.

"It's definitely the mental side of the game that I've been slacking on and that I need to improve on," Linsley said. "It's just the subtleties -- the outside zone step as opposed to the inside zone step, the differences between the aiming point are very subtle -- but they make a difference. That's what I've got to work on."