TODD PINKSTON didn't know right away that he would never suit up for another NFL game, when he tore his right Achilles' tendon early in Eagles training camp, on Aug. 5, 2005.

"I didn't know anything about it then," Pinkston, now a high school receivers' coach in Petal, Miss., said yesterday when a reporter phoned. Eagles safety Marlin Jackson ruptured an Achilles' at Tuesday's OTA, triggering memories of Pinkston's demise, which occurred as the wideout prepared for what would have been his sixth pro season. The injury is one of the most serious that NFL players face; many don't get back to where they were before they were hurt.

Pinkston quickly learned what a problem an Achilles' injury could be for a player who depends on speed. At training camp a year later, he struggled, and the Eagles cut him before the season. Pinkston then was picked up by the Vikings, but lasted there only 6 days. The next year he went to camp with the Redskins, feeling he was finally back to form, but was cut again, and retired.

Pinkston said the reason why an Achilles' injury is so dangerous to a skill-position player is simple. "Because, when you're running, that's what you're using," he said. "It comes back, but it takes 8 to 12 months, minimum."

Even after 12 months, Pinkston was missing "my burst," he said. "My burst of speed. I got it back eventually, but it was too late then."

"I wouldn't wish that [injury] on anybody," Pinkston said.

Pinkston stressed that he wasn't predicting a dire outcome for Jackson, who has suffered ACL tears each of the past two seasons. Some players do make it all the way back - among them former Eagles defensive end Derrick Burgess, who suffered a season-ending foot injury in the 2002 opener, and missed 2003 with an Achilles', but later made two Pro Bowls; and former Eagles linebacker Takeo Spikes, who suffered his Achilles' tear playing for Buffalo less than 2 months after Pinkston went down, but came back and is still playing in the NFL with San Francisco. Burgess was a strong contributor to the Birds' Super Bowl drive in 2004, the year after his Achilles' injury. Spikes struggled when he returned to the Bills in 2006, and ended up being traded to the Eagles the next offseason.

"Everybody's different, everybody's body is different," Pinkston said. He said he spoke to Spikes after Spikes' injury about the need to be patient - something Jackson probably does not want to hear right now after 2 years of knee rehab.

"I went in there yesterday - you could see he was just heartbroken," Eagles strong safety Quintin Mikell said, when asked about Jackson. "In this sport, you've just got to keep fighting. Hopefully, he'll get another shot, and just go from there."

Jackson was not available for comment. Agent Doug Hendrickson has not responded to inquiries.

There is virtually no chance Jackson will play for the Eagles this season. Yesterday's starting free safety for OTAs was second-round rookie Nate Allen, who was drafted to be an impact player, but now faces an accelerated learning curve.

"Coming in in this defense, he's got his work cut out for him. You saw the struggles we had last year [with free agent Sean Jones, second-year man Quintin Demps and rookie Macho Harris]. At the same time, he's got the right attitude, man; he's coming in focused, he's a smart guy. He's fast, he can make plays on the ball, he's got good size," Mikell said. "The good thing is, he's listening. He's taking everything we're giving him and he's trying to apply it on the field. I have a good feeling about him."

Mikell said "the speed of the game, the mental part of the game," is what will challenge any rookie safety the most. He said that with the extra reps Allen is getting now, "I think he's going to do fine with that, as long as he's focused and doesn't expect too much of himself."

Allen was asked how the news was delivered, that he would be working with the first unit.

"I saw it on the depth chart," Allen said. "I just knew I had to step in and keep the tempo going . . . It's a big challenge. I know I have to step my game up, and like I keep saying, not lose step with the first team, just keep it going. I just knew I couldn't hold the 'ones' back, I had to keep the tempo going."

Allen, 6-0, 210, said the Eagles' defensive playbook is "10 times, a hundred times" thicker than the one at South Florida. "I feel good about it, though," he said. "I'm just asking the veterans a lot of questions, asking the coaches. I feel good about what I'm doing."

Harris remained at corner yesterday, making Demps the other main option at free safety, though Allen seems to be the preferred candidate.

"I don't know what my role is, man, I know I'm on the boat," said Demps, who whiffed on the chance to succeed Brian Dawkins a year ago. "I'm not the captain of the boat, but I'm on the ship, and we're riding, we're going to the Super Bowl. That's what I'm trying to work at."

Demps acknowledges now that he was too cocky last year, that he put more energy into boasting than learning.

"I was trying to be a Pro Bowl safety instead of a starter," he said.

Demps is one of the vets Allen has sought out for advice.

"He's good, man, he's confident, very smart guy," Demps said. "But he's like every other rookie when they first come into the Eagles' defense - you're all over the place. But he's communicating, asking questions, that's the big thing."

For more Eagles coverage and opinion, read the Daily News' Eagles blog, Eagletarian, at www.eagletarian.com.