There is an important distinction that needs to be made.
Two versions of the Physically Unable to Perform List actually exists, but only to serve as sequential definitions. Players may be played on Active/PUP at the start of training camp, provided that they haven't practiced during any training camp practices -- and usually it'll happen before the first practice.
Though one deflects this label because players can come off that list whenever they're ready, it's required placement if someone is to be placed on the Reserve/PUP at the start of the regular season. At that point, you realize that the player will be ineligible for the first six weeks of the regular season, though benefits from a roster exemption for the 53-man roster.
Let's take a look a few players that could find themselves on PUP to start training camp.
TRAVELLE WHARTON: Suffered a season-ending knee injury during the first preseason game in 2012. Figures to challenge Clint Boling for the starting left guard job, but needs a clean bill of health before then. Could start training camp on the Active/PUP list, but the length of his stay can be anywhere from a day to a month. It just depends on the progress of his rehabilitation.
DRE KIRKPATRICK: Reports began surfacing before training camp last year that Kirkpatrick had suffered a broken knee, which was a simplistic version of Osgood Schlatter Syndrome. After a series of drawbacks and a stint on the Physically Unable to Perform List, Kirkpatrick made his NFL debut against the Broncos in week nine. A month later against the San Diego Chargers, Kirkpatrick suffered a concussion while his knee became problematic again. His season was done after that.
Nearly a year later and rehabilitation continues. Kirkpatrick underwent a procedure on his knee in January. After missing the team's offseason training program last month, Kirkpatrick is optimistic that he'll be ready for training camp. However, according to reports when minicamp broke, he had not been cleared by doctors.
BERNARD SCOTT: Maybe it's injury fatigue, or the dense cloud of durability questions, but the level of care that people have about Bernard Scott right now is microscopic. Resigned that he'll probably not make the 53-man roster, most simply shrug their shoulders with the former sixth-round pick.
Scott suffered a hand injury during training camp, forcing him to miss a handful of preseason games. An ankle injury forced him to miss another game, and as his luck would have it, during a 29-yard sprint against the Miami Dolphins, he suffered an ACL injury and was immediately placed on Injured Reserve. Questionable at best to be ready and may miss significant time.
CHRIS PRESSLEY: Still rehabilitating after suffering a season-ending knee injury against the Philadelphia Eagles, Pressley may miss the entire training camp period this year.
"It's a little frustrating because I'm injured right now, trying to recover from that,"Pressley told the Cincinnati Enquirer. "That's also to show I'm still fighting through, though. Nothing's going to keep me down like that. I'm coming back strong. I just think it's a cool full circle kind of thing (for me). I like it a lot."
ANDREW WHITWORTH: After playing in his first Pro Bowl this year, Andrew Whitworth had his knee scoped that dealt with "some scar tissue" which forced him to sit during the team's offseason training program. He's still on track to be ready for training camp and he's already warning everyone that he's never felt better.
"I thought it was a thing I could tough out, but it kind of wears on you," Whitworth says. "It was time to get it fixed and I feel great. I don't feel any different than the first day I played in the NFL."
TREVOR ROBINSON: Missed most of the team's offseason training program this year with a shoulder, Robinson is expected to be ready for training camp next week. Yet the team has depth and bodies to fill-in for Robinson even if the second-year interior lineman isn't ready. However, even if his arm was falling off, we don't see him missing time to compete for the starting center position against Kyle Cook.