CHARLOTTE – On Tuesday, head coach Ron Rivera addressed defensive tackle Kawann Short's absence from the third week of organized team activities.

"Well, because it's voluntary he decided not to come," Rivera said. "It's one of those things that he's got to decide what he wants to do. Because it's voluntary, we're not too concerned about it."

Short, a former second-round pick, is on the last year of his rookie contract. The Pro Bowler led the team with a career-high 11 sacks in 2015.

"We'll coach who is here, and that's the truth of the matter," Rivera said. "You'd love to have everybody here, but again, this is voluntary."

Rivera said he hasn't been given any indication if Short will take part in next week's three-day minicamp to close Phase Three of the offseason program.

"But next week is mandatory, so we're expecting him here," Rivera said. "He's under contract, so I'm not concerned. We expect him to do the mandatory things, we expect him to be where he's supposed to be."

Rivera also talked about two other defensive tackles.

On rookie Vernon Butler: "You're seeing a young guy that's learning and growing. He's doing some good things and making some mistakes. You see his athletic ability, you see his explosiveness. It's good that he's getting opportunities right now."

On Star Lotulelei: "He really looks like he's moving very well. You go back to the end of the season, those last four or five games into the playoffs, he looked really solid. He said it's the best his (previously injured) foot has felt in a long time."