"Was he out there?" he said with a laugh.
"No, I'm just kidding," Rivera continued. "We got on Charles pretty good [after we left camp in Spartanburg, South Carolina, on Tuesday]. He tweeted, 'Got through another camp.' And then the guys said, 'Well, you missed half of it.' We wanted to poke a little fun at Charles.
"But it is good to have him out there."
Johnson was held out of practice through most of training camp with a strained hamstring. Thursday was his first day back in almost two weeks. He says he wants to play in Sunday night's preseason game against the Kansas City Chiefs, and he hasn't been ruled out yet.
Whether it's worth the risk is the biggest question. Johnson is a seven-year veteran who doesn't really need the preseason to prepare. He reported in better shape than ever, having shed a few pounds from his 6-foot-2, 285-pound frame.
Nobody works harder before, during and after practice than the former Georgia star. He's probably the player Rivera and others point to the most when asked about the work ethic they want to see.
Having Johnson on the field for the opener at Tampa Bay is most important. Third on Carolina's all-time sack list with 54, he sets the tone for the defense even though he's one of the quietest players on a unit that was ranked second in the league last season.
This preseason, Johnson has been more outspoken as the Panthers look for leaders to replace left tackle Jordan Gross and others who were vocal last season. Players listen to him because they respect him.
"He has been more vocal than he has been, which I think is exciting," Rivera said. "When veteran guys who do things the right way step up in leadership roles, to me that means they understand and get it and take responsibility.
"You have to have that to be successful."
But mostly Johnson leads by example.
"I always want to be on the field," he said. "That's what I do. I like to work. I tried to get back as quickly as possible, because guys are always joking around with me, asking me when I'm going to come back on the field because they miss me.
"It's all good, though."