DENVER, CO - JANUARY 01: Linebacker Justin Houston #50 of the Kansas City Chiefs celebrates after sacking quarterback Tim Tebow #15 of the Denver Broncos at Sports Authority Field at Mile High on January 1, 2012 in Denver, Colorado. The Chiefs defeated the Broncos 7-3. (Photo by Doug Pensinger/Getty Images)
Actually, we do know why. The KC Star's Adam Teicher reports that Houston was sick. Presumably this isn't a big issue, just an illness, and he'll be back soon.
After Studebaker, the Chiefs have Cameron Sheffield and Gabe Miller. Both are unproven players who haven't seen much of the field since arriving in the NFL as Chiefs draft picks, Sheffield in 2009 and Miller in 2011.
We'll see if Houston returns to action today.