CINCINNATI -- Training camp may have started on a high note for the Cincinnati Bengals, but things have started going differently in the past five days.

As the physicality of the practices ramped up, injuries ramped up, too. Notably, receiver Marvin Jones has had a leg issue since Sunday, the first day players put on shoulder pads. That same afternoon, veteran defensive end Michael Johnson went down with a knee injury that could keep him out for the season opener.

Asked to address those injury concerns and others during a Thursday news conference, coach Marvin Lewis harped on how life must go on for the rest of the team.

"They don't exist until they're ready," Lewis said about the injured players. "That's the way we have to go about it with all these guys. They're in their rehab mode and that's where they are. They're in their rehab model until they come out and prove that they're ready to come back to practice. Then we'll make that determination about whether they're ready to play."

In addition to Jones and Johnson, the Bengals have been without linebackers Rey Maualuga and Vontaze Burfict all of camp. Maualuga was placed on the active/non-football injury list as camp opened with an undisclosed injury. He still has participated in conditioning drills on side practice fields.

So has Burfict. The fourth-year player joined Maualuga and a couple other players on the active/physically unable to perform list on the side fields Monday. Since then, he has been working through agility drills, as well as those to test his explosion and power as he starts trying to come back from offseason microfracture surgery. He spent the first three days of training camp doing that work inside Paul Brown Stadium on the newly installed indoor rehab space.

What could be gleaned from Burfict's appearances?

"That he was tired of being inside," Lewis said, chuckling. "He wanted to see the sun."

He won't be seeing it Thursday, as the Bengals were forced to cancel the day's open session for fans due to inclement weather. Instead of practicing on the outside practice fields adjacent to the stadium, they will work out inside it.

Lewis did eventually admit Burfict's appearance outdoors was a sign he's taking a step in the right direction.

"He just has to keep after it," Lewis said. "He's got a certain timeframe in his mind, but with that timeframe, you know, I see better than I hear. So when he's ready to get back and join the football team just like we talked about with Marvin Jones and Michael Johnson or whoever else it is, when we see that in my eyes, then he'll join the football team. But not before that."

With respect to Jones, a receiver who missed all of last season due to foot and ankle injuries, Lewis sounded a little less patient.

"Marvin's got to get to practice if Marvin wants to get to play," Lewis said. "The passing game, a lot of it's about timing and repetition and so forth. It just doesn't happen. You've got to work through it and practice and play and do those things. He's been a bit [behind] because he didn't have last season."