For the first time since DeSean Jackson exploded out of the chute as a rookie, the Eagles will find out what their team is like without the greyhound-quick wide receiver.

Because he has experienced post-concussion symptoms and perhaps because of the sharper focus on those types of injuries in the NFL, Jackson will not play tomorrow at Atlanta.

Eagles coach Andy Reid said that his leading receiver "felt pretty good" yesterday, but that he had experienced headaches as late as Thursday.

Jackson underwent testing yesterday, but the team had not received the results by the time Reid met with reporters. Still, the team's medical staff had seen enough to keep Jackson, who hadn't practiced all week, on the shelf.

"I just don't think he's ready yet," Reid said. "We'll wait until next week and see how he does."

Jackson joins a casualty list that still includes Akeem Jordan, who has been out with a knee injury. The linebacker, who saw his practice time decrease to the point where he did not work out yesterday, will be out for the fourth straight game.

"He's just not quite ready," Reid said. "He had a little bit of swelling [Thursday], so we just backed off of him."

With Jordan inactive, the defense will once again have to rely on the old (Jeremiah Trotter) and the helpless (Joe Mays) at middle linebacker. The defense has been in survival mode the last two weeks, but has produced stops when they counted most.

Not having Jackson, though, will hurt. He's the Eagles' best deep threat - and maybe even the NFL's - and his absence creates holes not easily filled.

Who will stretch the Falcons' secondary and who will return punts? Jeremy Maclin is the answer to both questions, although past performance would seem to indicate that he'll have more success in the former role.

"I sure hope that he's not just strictly a possession receiver," offensive coordinator Marty Mornhinweg said. "He's shown that he can go down the field and make great catches and outrun some people."

Jackson is averaging 17.5 yards a catch and Maclin 12.9. But in the one game this season in which Jackson was blanketed and held to just one reception, Maclin caught six passes at 23.7 yards a pop. Since the Tampa Bay game, though, the rookie has run more routes underneath.

Maclin said that he wouldn't necessarily step into Jackson's routes, but that he expected to see more passes from quarterback Donovan McNabb. All of that, however, will depend on the success of the receivers opposite Maclin - Jason Avant and Reggie Brown.

Avant normally plays in the slot and is better suited to the role, and Brown had once started at flanker and is more natural on the outside, but they will be interchangeable.

Avant is "looking forward to playing on the outside," Reid said. "Most of the work he does is on the inside, so I know he's looking forward to doing that. Reggie Brown will also get some snaps in there."

Brown has seen his star fall over the last two seasons.

Injuries had something to do with last year's descent. Jackson's meteoric rise - which began when he went over 100 yards receiving and returned a punt 60 yards in his first game as a rookie - also played a part.

Brown has "been fighting like heck to get another opportunity, and it looks like he's going to have plenty of opportunities this week," Mornhinweg said.

Brown, who has only three catches this season, declined several requests to be interviewed this week.

Jackson's absence also resulted in Jordan Norwood's call-up from the practice squad and the likelihood that the Penn State product would be the Eagles' fourth receiver in his first NFL game.

"If we have four up, then four play," Reid said. "There's a chance Norwood is up this week, and we'll just see how things go for him. We have confidence in him."

Reid has had to rely on rookies, especially on offense, more than at any other time in his 11 years with the Eagles.

Running back LeSean McCoy has done yeoman's work filling in for Brian Westbrook, who will miss his fifth game because of his two concussions. And Maclin, who was clutch with Avant after Jackson was knocked out last week against Washington, has been steady.

If there has been a quibble with Maclin, a No. 1 draft pick, it has been his return work. He lost the kick returner's job a few weeks back and has fielded three punts for minus-4 yards.

"It's not 11 walk-ons out there anymore," said Maclin, a good return man at Missouri. "It's guys who are getting paid a lot of money to go out there and do their job."

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