The Marlin Jackson experiment lasted all of 83 days.

Less than three months after the Eagles signed the free agent safety to an incentive-laden two-year contract, Jackson ruptured the Achilles tendon in his right leg during practice Tuesday.

His season is likely over long before it ever started, and his career appears to be in jeopardy. The Eagles did not say if Jackson would undergo surgery, but he also has the added misfortune of two recent ligament tears in both knees that made him such a risky signing in the first place.

As dramatic as Jackson's injury was - because of both his teammate's instant reaction and his upon learning the news - it may not even be the most significant. Guard Todd Herremans did not practice in the first post-minicamp workout for the entire team because of a left foot injury.

The team did not specify the problem, but Herremans had surgery on the same foot last August when he suffered a stress facture during training camp. He missed the first five games of the season but returned for the remainder of the year.

The start of the season is still more than three months away, but not having Herremans around could affect an offensive line that is already missing center Jamaal Jackson and trying to incorporate last season's injury poster player, Stacy Andrews, in at right guard.

Herremans apparently didn't think much of the injury. After practice, he tweeted, "Great workout this morning. Trying to figure out how to enjoy the weather."

While Jackson was running with the first team during drills, his opening the season as the starting free safety was not a foregone conclusion. When the Eagles signed the former Colts cornerback on March 10, they said he would compete for the open free safety spot that a trio of candidates could not hold down in 2009.

The deal, though, was loaded with incentives based on Jackson's health and his starting. He tore the anterior cruciate ligament in his left knee last October and in his right knee the previous year and had played in only 11 games in the last two seasons.

At the time, Eagles general manager Howie Roseman called the team's first free agent acquisition of the off-season a "risk-reward." A team source said Tuesday that they viewed the signing as "virtually no risk."

Still, the Eagles will have to adjust. They have no plans to add another safety, according to a source, not even the Rams' Oshiomogho Atogwe, who could become an unrestricted free agent on Wednesday. Instead, they will look to three young players to fill the hole.

Macho Harris was recently moved to cornerback, but the second-year player has the most playing experience, having started at free safety for seven games last season.

"Whatever the coaches may need me to help the team," Harris said.

Quintin Demps lost the job prior to the start of last season and found his way down the depth chart after injuries and some immature comments landed him there.

"Last year, I talked too much," Demps said. "I was cocky, but I was cocky with my mouth instead of just showing it."

Demps can also return kicks, which could help his cause. But he's been playing mostly second team strong safety behind Quintin Mikell – although that shouldn't matter much since both safety positions are interchangeable – and he's behind rookie Nate Allen, who stepped in for Jackson after he went down.

The Eagles selected Allen in the second round of April's draft, so it's fair to say they viewed him as the eventual starter. This may be earlier than they expected, however.

"I just know I got a lot of work to do," Allen said. "I got this saying, 'Once you get [the playbook] down, it gives you the opportunity to play fast.' "

Jackson didn't look so fast in the first two weeks of spring practices. He was running with an obvious limp early on – an observation he later dismissed – and was held out of various team drills. But there he was, practicing inside the indoor facility on the turf the team often keeps players returning from injury off of.

During a 7-on-7 session, Jackson ran after receiver Jeremy Maclin and pulled up lame without contact. He removed his gloves immediately and slammed them on the artificial carpet. The training staff helped him to the sidelines where head trainer Rick Burkholder apparently told him the severity of the injury.

Jackson waived his hands up in the air, shed his jersey and left the field sobbing loudly. He was eventually carted away from the facility, head in hands.

"It's definitely tough. . . . There was a moment of silence," Harris said. "With a situation like that all you can do is pray. Ask God to step in his life for understanding and guidance. Man, I could feel his pain."

Contact staff writer Jeff McLane at 215-854-4745 or jmclane@phillynews.com.