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JC Tretter will miss at least the first eight weeks of the 2014 season after being placed on injured reserve with designation to return Wednesday.

The second-year center already was declared out for Thursday's regular-season opener against Seattle with the left knee injury he sustained in the Green Bay Packers' third preseason game against Oakland on Aug. 22.

Tretter's move to injured reserve opened a roster spot for center Garth Gerhart, who was promoted from the practice squad, but prevents Tretter from playing for eight weeks. He would be eligible to resume practice after six and return Nov. 9 against Chicago.

NFL teams first were able to start placing players on injured reserve with a recall designation on Tuesday at 3 p.m. (CDT). Each team is able to recall one player.

Gerhart, 25, spent part of the past two seasons on the Packers' practice squad. He was cut at the end of camp last summer, but returned to the practice squad for the final 12 weeks of last season. The younger brother of Jacksonville running back Toby Gerhart, the 6-foot-1, 310-pound reserve provides added depth behind rookie Corey Linsley, who'll start his first NFL game at center against the Seahawks.

To fill Gerhart's spot on the practice squad, the Packers signed former Louisiana Monroe offensive lineman Josh Allen, a 6-foot-3, 312-pound rookie who was released by Tampa Bay during the final wave of cuts. He started 42 of his 47 games in college.

Since it was approved in 2012, the Packers have tabbed running back Cedric Benson and receiver Randall Cobb as injury designations. Benson never played again in 2012, but Cobb returned for last year's regular-season finale in Chicago.