After being hurt in the third preseason game, Hillenmeyer was cleared to return and played in Chicago's opener against Detroit on Sunday but was removed from the game at halftime after he became ill.
Hillenmeyer filled in at middle linebacker for injured Brian Urlacher last season and made a career-high 105 tackles. In his seven years with the Bears, he has made 69 starts.
"Player health and safety has and always will be a priority with our organization," 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 it is difficult knowing his season has ended.
"I have mixed emotions," Hillenmeyer said. "I respect the decision of Jerry 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," he said.
"But when it is my season that is cut short, it makes the complexity of the issues really hit home."
Copyright 2010 by The Associated Press