There was a time, easy to forget now, that Nolan Carroll and JaCorey Shepherd were supposed to be something more than invisible. They sit two lockers apart from each other in a corner of the NovaCare Complex locker room. They were sitting there Tuesday, minutes before the first practice of the Eagles' mandatory minicamp this week, their first practice since an injury ended each of their seasons last year.

Carroll was one of the Eagles' starting cornerbacks last season, for the team's first 11 games, until he broke his right ankle Nov. 27 against the Detroit Lions. Even though he was a rookie sixth-round draft pick (from the University of Kansas), Shepherd was supposed to be the Eagles' primary nickel cornerback until he tore the anterior cruciate ligament in his right knee during an Aug. 10 practice. So much is different since each was last on the field: a new head coach in Doug Pederson, a new defensive coordinator in Jim Schwartz, a new defensive scheme, an old reason to arrive at the practice facility every day - to practice.

That's the worst part for any NFL player who has suffered a season-ending injury. "Actually being here," Shepherd said. "I'm here physically, but being here mentally is tough. It's tough to sit in meetings for a whole nine months and prepare like you're playing, like you're going out there to practice, then going home and watching film, taking notes like I'm actually playing." But knowing that he would not play. And knowing that he wouldn't enjoy whatever benefits he might reap from his preparation until the following year. And knowing that his teammates would look right through him, because once a player cannot play, in a sense he ceases to exist, at least until he is healthy again.

"It's like they don't even see you," said Carroll, who had two interceptions and defended two passes last season before breaking his ankle, his first major injury in his six years in the league. "I understand it, but at the same time it gets to you a little. I'm still working. I'm still part of the team. I'm coming in. But guys aren't really talking to me.

"You've got to try to find some way to stay into it. It's hard because there are so many things in your head. Am I going to come back right? You're in the film room, and you see mistakes guys are making. You're thinking to yourself, 'I wouldn't make that mistake.' You see certain things happen, and you're like, 'I knew this was coming.' You just see yourself doing certain things when you're watching film, and it's frustrating."

Pederson said Tuesday that he, Schwartz, and the Eagles' coaching staff would limit the number and rigorousness of Shepherd's and Carroll's repetitions this week, to gauge where the two of them are physically. "Once we get into training camp and get these guys 100 percent," Pederson said, "we'll be able to see more where these guys fit."

It's a prudent course of action, given that the Eagles' entire group of cornerbacks qualifies as a significant question mark. Schwartz's system calls on his cornerbacks to play a lot of one-on-one man coverage, and nothing is settled yet among Shepherd, Carroll, Eric Rowe, Leodis McKelvin and Ron Brooks. In the best-case scenario, both Carroll and Shepherd recover fully and fall back into their previous roles. Carroll would be a returning starter, and Shepherd would be the functional equivalent of the Eagles' drafting a cornerback in this year's draft, except that he would have had a year-long apprenticeship in the NFL.

"You use that time to learn how to learn," Shepherd said. "That's not something you can just come in and know how to do. It takes time. It's a process to learn how to study, what to look for. Now that I am able to play, that's one less thing I have to worry about."

There is so much less to worry about, for both him and Carroll, now that they are able to play. They are professional athletes, yes, and they are paid well, of course, but this is their job. This is their work, and work is supposed to be fulfilling. When it is, it's a rare and good thing, a virtue, and it's easy to forget that when you don't have it anymore, when you're sitting in the same place day after day and no one can see you.

msielski@phillynews.com

@MikeSielski