Posluszny was listed as questionable on Friday's injury report because of the concussion he suffered in the fourth quarter of last Sunday's victory over the Tennessee Titans.
But Posluszny failed to pass concussion tests on Saturday.
Posluszny is the team's best defensive player and responsible for three of the defense's 12 turnovers. He has a team-high 88 tackles, two interceptions, eight pass breakups, two forced fumbles and one fumble recovery.
Outside linebacker Russell Allen, who has started 39 games in his five seasons, will start in place of Posluszny. Allen set career highs in tackles (201) and pass breakups (eight) in 2012 and has 40 tackles and two fumble recoveries.
"Fortunately we've had all week of practice with Russell in there, and he's done a nice job this whole week," Jaguars head coach Gus Bradley said Friday. "He was thrown into a little tougher situation last week with the limited amount of reps, so at least this week he got multiple reps.
"That's where Russell's so important to us -- in other ways, too -- but the flexibility to play multiple positions."
The Jaguars (1-8) also promoted defensive end Ryan Davis from the practice squad to the 53-man roster. Davis spent the first 10 weeks and most of last season on Jacksonville's practice squad.
Information from The Associated Press was used in this report.