Marlin Jackson went up to defend a pass down the middle of the field, came down on the artificial turf inside the practice facility at the NovaCare Complex and the whole picture in the defensive backfield changed. For how long, we don't know, but a ruptured Achilles tendon sounds like a season-long injury. Jackson, signed in the off-season as a low-risk, high-reward player, reacted the way players react when they know they are seriously hurt.

Jackson immediately clutched his right leg, rocked back and forth, and then took off his gloves and threw them to the ground. He was helped off the field and, as the silence grew inside the practice area, was in clear agony. Jackson was driven by cart to the athletic training room at the NovaCare Complex, head down, clearly distraught.

The initial reaction from onlookers was that Jackson, who has torn each anterior cruciate ligament in the last two years, had done something severe to his right leg. The report after practice was that Jackson had suffered a right ankle injury. Later on Tuesday afternoon, after Jackson had an MRI, the official report was that he suffered a ruptured right Achilles tendon, and, well, he is cursed with yet another injury.

It was tough to watch Jackson go down. He had made great progress since signing with the Eagles and he looked good in practice, good enough to run with the starters at the free safety position. Jackson was, by all accounts, on the right road back after some devastating injuries while a Colt the last two seasons.

But now the Eagles are going into the immediate practice future with Nate Allen and Quintin Demps bumping up the depth chart. Kurt Coleman, a seventh-round draft pick from Ohio State, reports on June 9, which means he will have all of two days to practice before the team breaks for training camp.

The Eagles knew the injury risk there when they signed Jackson, and that is why they used the second-round draft pick on Allen. Every day becomes more and more critical for Allen, and for Demps, who knows he needs to rebound after his disappointing 2009 campaign.

"It's tough for Marlin. I have to get in the playbook and keep preparing like I have been," said Allen. "I'm picking up everything pretty well, learning a lot from the older guys, asking a lot of questions. I'm pretty happy with where I'm at right now, but I can always improve."

Allen will get more reps, and in fact, could very well work with the first-team defense when practice gets underway on Wednesday. He was the 37th pick in April's draft and he was a four-year starter at South Florida and it is clear that he has fine ball skills and a good head on his shoulders and that, at first glance, he runs plenty well enough to play at a high level in this league.

The Eagles see the safety positions as interchangeable, to a degree, so Allen has a lot to learn. The volume of the defensive playbook is the challenge for Allen, so these next seven practices are extremely critical. He is a young safety whom the Eagles see as the future. Depending on his progress in a short period of time, Allen could very well be the present, too.

Demps will have something to say about that. He knows last year was a downer. After running with the first team in the spring, Demps lost his job as the starting free safety in the summer and then, by season's end, was a healthy scratch. It was a significant slide for a former fourth-round draft pick who expected to be the starter last season.

With a new year comes new opportunity. Demps says he has grown up, says he has matured, says he is eager to prove to the coaches that he deserves another long look. He'll get it.

And he says he is ready for that chance.

"I've grown up. I've matured. This has been a test for me. I know there is always going to be a lot of competition and I have to be prepared for that," he said. "It wasn't easy for me last year, but I have nobody to blame but myself. This is a new me. I'm deeper in my faith and I'm working harder than I have ever worked to make myself a better player."

On a day when the defense looked so fast, and so aggressive, and got its hands on so many balls, the excitement was tempered by Jackson's injury. And for those chilling couple of minutes when he was down on the ground and the normally-raucous practice environment went stone silent, the reminder of "one play away" was very much there.

Things change so quickly in the NFL. Two minutes after Jackson left practice the tempo was there again, the energy was present, and Demps made an interception and danced down the far side.

You don't stay in the past in the NFL. You move forward and you find out who else is ready to accept the challenge.

"You can't help but get better every day out here," said Allen. "We have a lot of work and if you apply yourself, you are going to see the results. Positive results. That's what I am looking for every time I step on the field."