But on a positive note, the club announced receiver Brandon Marshall -- who missed practice all week due to a sore right ankle -- is active against the Packers. He caught a touchdown pass from Jay Cutler on the Bears' opening drive.
Allen is suffering from pneumonia, a league source has confirmed. Fox Sports originally reported the nature of Allen's illness and also reported that Allen, who is listed at 270, lost at least 15 pounds.
The Bears kept Allen away from the team's facilities for the majority of the week to let him recuperate. And though coach Marc Trestman expressed optimism Friday about the veteran's prospects, he wasn't able to recover in time to face the Packers. Sunday's game is the first he has missed since 2007.
"Obviously it's an opportunity, but at the same time, Jared would be missed," Young said on Friday. "We're professionals here. When one guy goes down, we've got to look for guys to step up and be effective."
In other injury news, cornerback Sherrick McManis (quadriceps), linebacker Shea McClellin (hand), center Roberto Garza (ankle), guard Matt Slauson (ankle), offensive tackle Charles Leno, and defensive tackle Jeremiah Ratliff (concussion) were listed among the team's inactives.