IF YOU WANTED to make a list of things that haven't gone the way the Eagles anticipated this season, the offensive-line situation would have to go right there at the top.

Yeah, linebacking would be a close second, but which unit is more critical to success?

Now, the o-line's most stable spot just got destabilized. Steady center Jamaal Jackson will miss the rest of the season with a torn left anterior cruciate ligament. Jackson told the Daily News he will visit Dr. James Andrews in Alabama within the next few days.

"I'm heartbroken that I can't help my teammates win a divisional title and possibly a Super Bowl," Jackson said in a text message, when asked whether he wanted to say anything to Eagles fans. "But I thank [the fans] for all the support that they have showed me over the years, and somehow, some way I'll be back on the field in the future!"

The timing couldn't be much worse, with the Birds preparing to play at Dallas Sunday for the NFC East title. The Cowboys' defense is strong up front, where the Eagles now have Nick Cole at center and either Max Jean-Gilles or Stacy Andrews at right guard, where Cole had started since left guard Todd Herremans returned to the lineup from a foot injury, Oct. 26. The line has had the same five starters in the same spots since Nov. 22, when left tackle Jason Peters returned after missing a game with an ankle problem. Not coincidentally, the Eagles have won all six contests they've played in that span.

"We've lost a great center," fullback Leonard Weaver said last night on Comcast SportsNet's "Daily News Live." "A great player who's been playing awesome all year, our signalcaller [for protections]. It's going to be [important] for us to respond in the right way."

"It's obviously a big loss," coach Andy Reid said. Jackson has started the last 71 games (and five playoff games), ever since taking over for an injured Hank Fraley midway through the 2005 season. Jackson made the team as an undrafted free agent from Delaware State in 2003. "He's a good football player. We're fortunate enough to have Nick here, who's a center, who's been playing guard for us . . . This isn't a new territory for him . . . he and Donovan are on the same page. He can make all the calls, he can do the silent counts, he can do the hard count, he knows all of that stuff, that's not a new area for him. And then, [guard Mike] McGlynn would be his backup there. [He's] another guy that has been practicing that position, played it in the preseason, and could step in there if he needed to."

Offensive-line coach Juan Castillo declined to comment, through a team spokesman.

Jean-Gilles has been the line's top reserve lately; Andrews hasn't been active since starting and playing poorly against San Diego Nov. 15. In very limited action, he has been extremely prone to making false starts and surrending sacks.

But Andrews is the guy the Eagles gave a 6-year, $38.7 million contract to last offseason, as he was recovering from ACL surgery. Theoretically, he has been refining his technique and gaining confidence in his rebuilt knee while not playing lately.

"Stacy is in the mix there," Reid said. "Max was obviously the guy that stepped in and played" Sunday after Jackson went down, apparently after having stepped wrong, with about 5 1/2 minutes left in the first quarter.

The line definitely was less cohesive after Jackson left the Denver game. Cole, who made the team as a backup center in 2006 but hadn't played the position this season, caused a few false starts and once snapped the ball to Donovan McNabb prematurely, Reid acknowledged, with the Birds backed up to their 1. The Broncos also got strong second-half pass rush pressure.

"It wasn't too tough. It was [more a problem] of getting back in the groove, getting used to it," Cole said after the game. "Making calls, making sure everybody's on point. We're used to [shuffling]. We've got guys ready to step in and do what they have to do."

McNabb noted after the game that he and Cole have some work to do this week to smooth out their operation. Reid said yesterday that the week of practice should take care of that.

"Nick is a good athlete," Reid said. "He looks like a big guy, so when you say he's a good athlete, it's hard to picture, but he is. He has great feet and very good hands and very good quickness."

The Eagles have been without their projected right tackle, Shawn Andrews, all season because of a back injury. They played the first five games without Herremans - who might be their top performer since his return - and have seen Peters leave three games with injuries. Cole hasn't missed any games, but was severely hobbled with a knee injury earlier in the season.

Birdseed

The Eagles brought back offensive lineman Greg Isdaner, from Episcopal Academy, to their practice squad. They probably will elevate practice-squad lineman Dallas Reynolds when they IR Jamaal Jackson . . . Andy Reid said Donovan McNabb, who seemed to have tweaked a hamstring near the end of his 27-yard run Sunday, told him "his hamstring actually feels pretty good" . . . Quintin Demps was to undergo an MRI exam yesterday on his injured ankle . . . Reid said Michael Vick's bruised quad is feeling better. *

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