The Colts had an idea of who they wanted to start in Mathis' place, but nothing was set after it was announced by the NFL that Mathis would be suspended.
After a little more than a week of training camp, the Colts have their answer, and it's the player they thought it would be.
Second-year linebacker Bjoern Werner, the Colts' first-round pick in 2013, has earned a lot of praise for his improved pass rush and instincts to react quickly and bat passes down at the line of scrimmage.
"He knows what he's doing," Mathis said. "As a rookie you're trying to find your way and just trying to see what the game is all about and what not. This year, he's knows what he's doing; he knows the plays."
There was plenty of unease inside the Colts organization when they learned Mathis would be suspended the first four games of the season for taking a banned substance. The coaching staff sat down and talked to Werner about starting, but the second-year linebacker had already taken the mental approach that he was going to have to step his game up even more.
Werner was already three months into his offseason workout regime. His rookie season didn't go as planned for him, so he knew he had to get stronger, quicker and get a better understanding of the defensive scheme. He finished with 15 tackles and 2.5 sacks last season.
"The learning curve never stops," Werner said. "You learn every year more and more. The more you play, the more experience you get, and the more comfortable you get. I try to pick up as much as possible even if I know the whole playbook, there is always different formations coming up and you have to pick it up. I have to know them."
Like Mathis, Werner had to adapt from being in the down position as a defensive end at Florida State to being a linebacker in coach Chuck Pagano's 3-4 defensive scheme. Werner is now playing the rush linebacker with Mathis out, and that makes a significant difference, according to Mathis.
"Rush linebacker is a heaven sin position because it allows you to be a playmaker and [Werner] kind went the same route I went," Mathis said. "He played [strong side linebacker] first to learn how to really play linebacker and then moved to rush where you can put two and two together. It taught him how to play linebacker at [strong side] and then how to be a playmaker at rush."
Werner's improved play was noticable early in camp. He went through a stretch one day where he blocked an Andrew Luck pass that ended up being intercepted and then intercepted a pass that was also blocked at the line of scrimmage.
"He's miles ahead of where he's ever been at any point since he's been here," Pagano said. "He's got a really good grasp of the defense now. So whether he's dropping in coverage, or rushing the passer, or setting the edge in the run game, he's doing really well. You just keep moving in the right direction, getting better every single day and be that guy we need him to be at the start of the season."