Crennel had hung on to the coordinator's title after replacing Todd Haley, who was fired as head coach Dec. 12. But after going 2-1 the last three games of last season, the Chiefs have slogged to a 1-7 mark this year.
"I have decided that I am going to spend more time with the whole team and less with the defense," Crennel said. "We're trying to get this thing right and trying to get it turned. We have eight games to play. It's an eight-game season for us, so we're going to try and do everything we can to come out these eight games on the winning ledger."
Kansas City's defense ranks 29th in points allowed per game (30) and 17th in yards allowed (347.5) per game.
Also Monday, the Chiefs waived cornerback Stanford Routt and signed defensive tackle Shaun Smith.
Routt had signed a three-year, $18 million contract as a free agent in the off-season. He had 21 tackles and two interceptions over seven games this season.
Smith had 25 tackles and a sack in 15 games for Tennessee last season but was cut in August and hasn't played this season.