That's because he's dealing with a possible concussion, and though he's been cleared to practice -- and did so on a limited basis on Thursday -- the team isn't sure if he'll be cleared to play Sunday vs. the Raiders, according to ESPN.com.
Houston has 8.5 sacks on the season, just behind Robert Mathis' league-leading 9.5, but he reportedly took a shot to the head last week vs. the Titans. Houston didn't practice Wednesday, and though he reportedly hasn't shown any signs of a concussion, the team is being cautious with him.
Certainly, the Chiefs would miss Houston, but Kansas City also has one of the best front-sevens in the league, so his absence probably wouldn't be too profound. If Houston can't play, look for Frank Zombo to take his place in the lineup.