WHEN A PLAYER tears an ACL or ruptures an Achilles' tendon or breaks his collarbone, no one questions his toughness or his desire. No one sits him down and gives him the you-gotta-play-hurt speech.
None of his coaches openly suggest to reporters that he needs to get back on the field sooner rather than later if he hopes to secure a spot on the season-opening roster.
Which brings us to Chris Polk.
The third-year Eagles running back didn't shred his ACL or Achilles' or shatter his collarbone. He suffered a Grade 2 hamstring tear on the second day of training camp last month.
"I was running, trying to make a real hard cut," Polk said yesterday. "I heard a pop. I didn't know the severity of it, but I knew something wasn't right. It locked, tightened up. There was bruising and I just felt like my world ended that day."
Polk, who was expected to be the team's third back behind LeSean McCoy and Darren Sproles, hasn't practiced since that day. Didn't play in either of the Eagles' first two preseason games. Won't play tomorrow night at the Linc against the Steelers.
Polk has been running on the side at practice. Says the hamstring is feeling better, but still isn't 100 percent.
He is afraid that if he returns too soon, he will aggravate it again and be sidelined even longer. But he also knows that if he doesn't return to practice very, very soon, he's probably not going to make the team.
And if he didn't know that, head coach Chip Kelly and offensive coordinator Pat Shurmur spelled it out loud and clear for him this week.
Asked how big a factor Polk's nearly monthlong absence from practice is as far as his chances of making the season-opening roster, Kelly said, "It's a big factor. You've got to be able to play. It's tough for us to say we know what you can do. You also have to be able to show it."
Said Shurmur: "It's like going to school and skipping class and all of a sudden, you have a final exam. You certainly might pass it, but there is a chance that you won't do as well as if you'd been there every day.
"Durability is something we consider. Guys get injured for different reasons, but we'd sure like to see him out there."
As Polk correctly pointed out yesterday, the NFL is a what-can-you-do-for-me-now league, especially when you were an undrafted free agent with 11 career carries and four receptions.
But Kelly and the Eagles thought enough of Polk in May to trade away backup running back Bryce Brown to the Bills. With Sproles expected to be used primarily as a receiver out of the backfield, Polk figured to get much of the backup ball-carrying chores behind McCoy.
"It's definitely frustrating," Polk said. "I mean, they got rid of Bryce. So they were counting on me. Especially in camp. I see this as opportunities that I missed to get better."
McCoy had a league-high 314 rushing attempts last season. If he continues with that kind of workload, he might win another rushing title or two, but he'll also be roadkill before he turns 30.
The coaches have talked up Sproles' running ability. But he's 31 and will be used primarily as a pass-catcher out of the backfield and as a punt and kick returner. In his last three seasons with the Saints, he carried the ball a total of 188 times.
That said, the Eagles likely will carry four running backs on their 53-man roster. Initially, it figured to be McCoy, Sproles, Polk and Matthew Tucker.
But undrafted rookie Henry Josey played himself into the conversation against New England last week, rushing for 56 yards on eight carries and catching a 27-yard touchdown pass.
Last night, the Eagles acquired another running back, Kenjon Barner, in a trade with the Carolina Panthers in exchange for a 2015 conditional seventh-round pick.
The league's new practice- squad rules, which were announced this week, also give the Eagles the option of cutting Polk and signing him to their practice squad.
Practice squads used to be limited to players who played in no more than eight NFL games. Under the new rules, each team can sign up to two players to the practice squad who have earned no more than two accrued seasons of free-agency credit.
Of course, the risk of putting a player on the practice squad is that he could be signed by another team.
"I'm close," Polk said. "I'm going to definitely practice next week."
Even if the hamstring isn't 100 percent?
"I got to," he said. "I hope I'll be 100 percent. It started feeling good today. But even if I'm not, I'll still go for it. I've missed too much. Even if it's not full-speed, I've got to show them that, hey, I'm here. So I'm going to do it."
Polk worked his butt off during the offseason. He practically lived at the NovaCare Complex. Got there every day at 6 a.m. Worked out. Trained. Watched film until he was blurry-eyed. Dropped 12 pounds so that he could better handle Kelly's uptempo offense.
"We talk a lot about sports science," he said. "When I was practicing, you look at my science numbers. I was off the charts.
"When I'm not out there, there is no one working harder than me. Because no one wants to be out there more than me. That's why it's so hard and frustrating now to be sitting this long. Plus, I feel like I'm leaving my boys out to dry. I'm just real anxious to get out there."
Polk's teammates understand his frustration. They understand the dilemma of wanting to go out there and try and play, but not wanting to risk reinjury and sitting out another month.
"Chris is in a rough spot right now," center Jason Kelce said. "Any time you have a pulled or torn muscle or something like that, there's not really a set number of days . They have a time frame on average of what it takes to get back.
"I know that guy is tough as hell. He played through a lot of injuries last year. It's got to be eating at him. Hopefully it doesn't, but it could end up costing him a spot."
Polk understands that. Like he said, it's a what-can-you-do-for-me-now league.
"There's nothing guaranteed," he said. "I wasn't drafted. I'm not LeSean McCoy or Nick Foles. I'm not guaranteed anything. I have to go out there and act like it's my last day."
On Twitter: @Pdomo