UNDER JIM Johnson, the Eagles' late, much-beloved defensive coordinator, roles were well-defined and they rarely changed. Johnson particularly didn't like messing with his starting lineup during the season.

Sean McDermott, Johnson's successor, inherited Johnson's scheme, but not his reluctance to make changes. It's true that some of McDermott's tinkering has been forced by injuries - the linebacker rotation would have been a lot more stable, one assumes, if prototype middleman Stewart Bradley hadn't gone down for the season in August - but McDermott certainly seems determined to keep shuffling his pieces until the puzzle fits or the season ends, whichever comes first.

At safety, that has meant Quintin Demps, Macho Harris and Sean Jones, all getting most of the snaps at the same spot at different points since training camp began. In the defensive tackle rotation, it's meant more Antonio Dixon, less Trevor Laws. The defensive end spot opposite Trent Cole has been a turnstile (Victor Abiamiri, Juqua Parker, Jason Babin, Chris Clemons), but in all fairness, that was the one thing Johnson never seemed satisfied with, either.

The biggest change is in what McDermott has done with his linebackers this season, though. A few weeks ago, the thinking was that things would stabilize when weakside starter Akeem Jordan returned from a knee injury, but the thinking was wrong. McDermott seems to have decided he likes Jordan in the middle, alternating with Jeremiah Trotter - or more accurately, McDermott has decided he would rather play Will Witherspoon on the weakside than in the middle.

This week, faced with yet another dominant tight end in the 49ers' Vernon Davis, McDermott seems to have decided to make another change. Seventh-round rookie Moise Fokou said he will be tasked with wrangling Davis; its unclear how much incumbent starter Chris Gocong is scheduled to play, if at all.

"Both of them will play in this game, and that's what that is," coach Andy Reid said yesterday.

Reid answered another question, asked about the safety situation, that really applied to McDermott's overall philosophy.

"If you dress right now on this football team, then you are going to play. There is no greater evidence than on the defensive side," Reid said. "They've all got their roles and they're filling them well. They'll all have the chance to play, whether it is at safety, whether it is at corner, everybody plays."

Gocong was not available for comment.

Fokou said: "I'll gladly take the opportunity."

Fokou replaced Gocong Nov. 8 vs. Dallas when Gocong was sidelined with a quad injury. Fokou also played the SAM spot the next week, against San Diego, when Gocong filled in at middle linebacker.

Fokou and Davis are friends and former Maryland Terrapins whose college careers overlapped for one season. Fokou said he texted with Davis a few weeks ago, before he knew they would be butting heads.

Asked what he thinks the coaching staff is looking for, Fokou said: "Probably for this week, jammin' that tight end and kind of running with him a little bit."

Davis leads San Francisco with 66 catches for 815 yards and 11 touchdowns.

"If you scheme it the right way, I think anybody in the league can be taken away," Fokou said.

Fokou said he hasn't minded the ups and downs of the linebacker shuffle - some weeks he plays a lot in the defense, other weeks he only plays on special teams.

"It's my rookie year. I'm learning so much every week, and I'm just taking it all in. Whatever's best for the team, I'm here to do," he said.

Celek's back

Tight end Brent Celek was a limited practice participant yesterday. Both he and Andy Reid said Celek would play tomorrow, although Celek officially is listed as "questionable."

Celek, who already was playing with torn thumb ligaments, wrenched his back getting up out of a chair following a meeting last Sunday morning, fewer than 8 hours before the Eagles played the Giants. He took treatment and was able to play, but did not practice Wednesday or Thursday.

"It's just one of those freak things; it happened in a meeting," Celek said. "I was always planning on playing. I never thought in my mind that I couldn't, but I know at that point I was a little nervous.

"To play in this sport, you have to be tough. You're going to have little bumps and bruises and nicks and you've got to be able to play through 'em. Otherwise it's going to be tough to play too many games in the NFL. Everyone has injuries. You just have to play through it."

Birdseed

As expected, wide receiver Jeremy Maclin (foot) and running back Brian Westbrook (concussion) will not play. Andy Reid said Westbrook, a limited practice participant yesterday, will undergo neurological testing again early next week in hopes of playing Dec. 27, against Denver. Reid said Westbrook "actually feels very good" but did not have medical clearance to play this week. "We'll see about this next week," Reid said . . . Reid reiterated that wide receiver Kevin Curtis would take the field in Maclin's absence, Curtis' first action since Week 2. Reid said Reggie Brown would start opposite DeSean Jackson, presumably with Jason Avant in the slot role. Curtis, who had knee surgery Oct. 24, will be the fourth wideout . . . With the Bears' game at Baltimore tomorrow moved to 4:15 to take the pressure off anticipated snow cleanup in Baltimore, quite a few cities scheduled to see that game instead will find Fox showing them the 49ers and Eagles at 1, the network announced . . . The NFL confirmed that Giants safety Aaron Rouse was fined $5,000 for head-butting DeSean Jackson last Sunday. Jackson incurred a $2,000 fine for a uniform violation. *

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