The Bears revealed that linebacker Hunter Hillenmeyer suffered a concussion in the third preseason game, and they placed him on the injured reserve Tuesday.
Hillenmeyer missed the preseason finale but passed all the medical tests before the regular season, the team said.
But, he was removed from the season opener against the Detroit Lions at halftime after becoming ill.
Bears coach Lovie Smith didn't elaborate about Hillenmeyer's status, except to say he was ill on Sunday and Monday, and that the team was still evaluating him.
It's a big loss for the Bears since Hillenmeyer was a valuable backup, including last season at middle linebacker. Last season, Hillenmeyer posted a career-high 105 tackles.
"Player health and safety has and always will be a priority with our organization," Chicago Bears general manager Jerry Angelo said in a statement. "Unlike other injuries, there is no defined timetable for a concussion.
"After consulting with our doctors and Hunter, this was the best way to proceed. We will miss his contributions this year, but it is the right decision given Hunter's circumstances."
Hillenmeyer said he had "mixed emotions."
"I respect the decision of Jerry and Lovie to place me on injured reserve. Concussions by nature are harder to evaluate than other injuries," he said. "While this is probably in my best interest and the team's, it is hard to accept the fact that my season is over in one game.
"As someone who has been in the forefront of player advocacy of concussion awareness, I'm happy to see my team err on the side of caution. But when it is my season that is cut short, it makes the complexity of the issues really hit home."
Adding to the intrigue: Hillenmeyer is one of more than a dozen players who has agreed to donate his brain to a research group determined to studying the effects of head trauma to professional athletes.
Tim Shaw, the team's special teams ace, was released by the Bears and then claimed by the Tennessee Titans. But one candidate is Kevin Malast, who was with the Bears during the offseason and training camp.