Some backup jobs are more important than others.

There are backup positions such as quarterback and center that are important - quarterback certainly more so - only if the starter is injured. Otherwise the No. 2 guy could spend the entire season watching the No. 1 guy take all the snaps.

And then there are backup positions such as wide receiver and defensive end in which you play part time in a rotation. And if you contribute and play enough, you aren't really a "backup."

Last year, Brandon Graham was a backup only in name. He played 43 percent of the defensive snaps in the Eagles' three-man outside-linebacker rotation, and his increased production and playing time were a significant factor in the improvement of the unit from Season 1 to Season 2 under coach Chip Kelly.

But Graham is now a starter and the Eagles have been looking for his replacement. The search has been complicated by injury. Travis Long, who was supposed to be in the mix last season before he tore the anterior cruciate ligament in his knee, suffered the same injury again in this camp.

And then Thursday, Marcus Smith, who has been given every opportunity to claim the job and has shown progress after an ill-fated rookie season, left practice with a hamstring strain. The injury isn't likely to be serious, but he will have an MRI on Friday, and at the least it will set back the 2014 top pick in his quest.

That leaves Kelly with a collection of candidates, from older to younger, but no one that seemingly can make the sort of "backup" statement Graham did a year ago behind starters Connor Barwin and Trent Cole.

"I don't say it's a concern," Kelly said before practice and before Smith's injury. "It's trying to figure out who all those guys are. We know two proven ones in Brandon and Connor and then who is the third is really going to be the big question for us. That's what we're trying to find out."

Brad Jones, signed as a free agent after six seasons with the Packers, has worked primarily at inside linebacker as Kiko Alonso, DeMeco Ryans, and Mychal Kendricks deal with injuries. But of the Eagles' options he has the most experience at outside linebacker and would seem to be the primary candidate for the third spot, perhaps even if Smith returns.

Jones' first three seasons were at outside linebacker, and in 35 games and 13 starts he recorded 80 tackles and five sacks. His next three seasons were inside, and in 41 games and 23 starts he had 213 tackles and five sacks.

"I've done both and like both," Jones said. "I think each of them have their unique qualities. You love getting after the quarterback. Everybody loves doing that. But the vision at inside linebacker is really, really ideal. You can see the whole game. You can make plays."

Bryan Braman's spot on the 53-man roster is secure because of special teams. He plays all four core units and was arguably the Eagles' most valuable contributor last year. But he couldn't crack the defensive rotation. He already plays about 25 to 30 snaps a game on special teams. Could he handle the additional workload?

"I'm guessing if I'm at that third linebacker spot, depending on where Marcus is at when the season starts," Braman said, "it might add maybe 15, 20 plays a game."

Graham averaged 31 snaps a game last year after averaging 20 the year before. With Barwin and Cole averaging 63 and 53 a game in 2014 vs. 71 and 55 in 2013, the production from the outside-linebacker group increased dramatically.

The outside linebackers totaled 261/2 sacks, 431/2 tackles for loss, 59 hurries, and nine forced fumbles in 2014 vs. 16 sacks, 21 tackles for loss, 28 hurries, and five forced fumbles in 2013.

"I understand the benefit of not playing every snap," Barwin said. "There's been multiple years in my career where I've played every snap. I can do it, and if I have to do it I will. But obviously, we hope somebody will step up."

Lower on the depth chart, the Eagles have undrafted rookies Diaheem Watkins and Dasman McCullum. Deontae Skinner, who spent last season with the Patriots and started a game, was acquired earlier this week. But he has started off inside.

The wild card is Vinny Curry. The fourth-year defensive end has increasingly seen time at outside linebacker. Davis said last week that he was just "playing around" with Curry at linebacker and that, if out there, it would typically be as an edge rusher. But with Smith out he played a bunch of snaps there Thursday and frequently had to drop into coverage.

"A lot of guys are helping me back there." Curry said. Outside-linebackers coach Bill McGovern "is doing a great job teaching me things, just baby-feeding me little steps of where I need to be, where my drops need to be. And for the most part, things are coming together really good."

Barwin kept calling for Curry to work on dropping after practice, and after he spoke to reporters, Curry walked over.

"Vinny can do it without a doubt physically," Barwin said, "but he's just got to catch up mentally."