Hillenmeyer suffered a concussion in the third preseason game against Arizona and failed to make the trip to Cleveland for the fourth and final preseason contest.
According to the team, Hillenmeyer was cleared to play in the season opener versus the Lions, but left the game after becoming ill.
"I have mixed emotions," Hillenmeyer said in a statement released by the team. "I respect the decision of [general manager] Jerry [Angelo] and [coach] Lovie [Smith] to place me on injured reserve. Concussions by nature are harder to evaluate than other injuries. 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."
Bears teammate and fellow linebacker Pisa Tinoisamoa was caught off guard by the news when he appeared on ESPN 1000's "Football Night in Chicago."
"That's huge," Tinoisamoa said. "I'm kind of blown away, stunned about that. Hunter's a great person, very smart. You can tell last year how he was playing when he came in for Brian [Urlacher]. ... My condolences to Hunter. I know it's tough for him. I know it's tough, and you know with concussions now they're pretty sensitive about those things."
Hillenmeyer, the Bears' union representative, also suffered at least one other documented concussion earlier in his career with the Bears. The veteran linebacker made headlines recently by agreeing to donate his brain upon death to a research group dedicated to studying the effects of head trauma on professional athletes.
"Player health and safety has and always will be a priority with our organization," Angelo said. "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."
Jeff Dickerson covers the Bears for ESPNChicago.com and ESPN 1000.