From ESPN.com:
Pittsburgh Steelers safety Troy Polamalu will not play Sunday at Kansas City and is expected to be sidelined about a month with a strained ligament in his knee, a source close to the situation told ESPN's Adam Schefter.
Polamalu injured a knee ligament while tackling Cincinnati's Cedric Benson on Sunday. He missed four games after hurting a different ligament during Pittsburgh's opener.
Polamalu's recent injury will not require surgery.