The Kansas City Chiefs defense used to be notoriously bad. But in the last few weeks, it’s looked almost elite.

Two offseason additions — Chiefs safety Tyrann Mathieu and defensive coordinator Steve Spagnuolo — have been given a lot of credit for that improvement.

Over the last three games, Mathieu has totaled 15 tackles, two interceptions and a tackle for loss — while Spagnuolo’s defense has allowed an average of 14 points and 103 rushing yards per game.

Those are vast improvements from the start of the season, showing that the defense has improved as the season progressed. Speaking to the press on Thursday, Spagnuolo credited Mathieu for having an immediate impact on the team.

“A friend of mine... that worked with or coached [Mathieu], he said something to the effect of, ‘He changed the building the minute he walked through the door’... that kind of rang true,” he said. “He’s dynamic, there’s a lot of energy, guys gravitate to him, he’s serious about what he’s doing. When you’re a coach and you have a player like that, that helps.”

According to head coach Andy Reid, Spagnuolo — who won a Super Bowl ring leading the New York Giants defense in 2007 — also made an immediate impression.

“I think he has done a phenomenal job of teaching his defense in a short period of time, and then getting results from that,” said Reid. “He’s got the guys believing in that system. That’s hard to do when you switch over.

“There’s a trust. There’s a certain trust that’s built in with that. Then on top of that, they were able to hit the ground running, just knowing what they do.”

According to Mathieu, the connection between himself and Spagnuolo — both in practice and on the field — has also had a good effect.

“Me and Spags, I think we feed off of each other. I feel like I’m an extension of him on the football field. Just trying to understand what he wants to get done, how he sees us playing the game plan and just being the middle man trying to relay that to our guys so that they can understand it.”

After the departure of former defensive coordinator Bob Sutton — and longtime leaders like Eric Berry and Justin Houston — the Chiefs needed coaches and players to fill leadership roles. It seems that Spagnuolo and Mathieu have filled those roles perfectly — and the defense is hitting its stride just in time for the playoffs.