Pro Bowl linebacker LaVar Arrington underwent arthroscopic surgery on his right knee for a lateral meniscus tear yesterday and will likely be out for two to four weeks, leaving the Washington Redskins, the league's top-ranked defense, without one of its most dangerous players.

After his knee swelled Wednesday night, Arrington visited orthopedic surgeon James Andrews in his Birmingham office yesterday and the decision was made to trim a meniscus flap that was causing continued discomfort.

Arrington is expected to be on crutches for a week and will visit Andrews again next week for a progress report. Coach Joe Gibbs said that such surgery usually keeps a player sidelined for two to four weeks, but a formal timetable for Arrington's return has not been finalized.

Arrington, 26, joins a growing list of injured key players on the Redskins and his absence leaves the team without several starters for Monday's game against Dallas. Quarterback Mark Brunell has been unable to practice fully this week because of a strained left hamstring and could miss Monday's game, right tackle Jon Jansen was lost for the season in the preseason opener, middle linebacker Mike Barrow has yet to play a game because of tendinitis in his left knee and defensive end Phillip Daniels is out at least the next two weeks with a right groin tear.

"We're going to have to go until [Arrington] gets back," Gibbs said. "From time to time that happens up here. We hate it and I think LaVar sucked up his guts last week and played with it and we thought he did a heck of a job for us. It's just one of those things that happens and you need to get it taken care of. He's somebody we think is going to play for a long time, so we're going to make sure we take care of it."

Arrington dealt with a nagging left knee injury in the preseason and strained his right knee in the season-opening victory over Tampa Bay. Arrington missed practice for a spell last week because of the problem and reserve linebacker Lemar Marshall split time with him leading up to Sunday's game against the New York Giants. Arrington's knee was stimulated early Sunday morning and he played through the pain in the loss to the Giants. His knee felt better than it had in weeks when Arrington reported for treatment Wednesday -- a day off for players -- but the injury worsened overnight.

"[Wednesday] was the best it looked," Gibbs said. "Sometimes evidently with those things overnight it just really swelled up. So we felt like it was better to go ahead and get them to look at it . . . He should be okay, but it's too bad that thing swelled up on us."

Arrington, the second overall pick in the NFL draft in 2000, has missed only two NFL games (both in 2001) and is a vital cog in assistant head coach Gregg Williams's defense. He was switched from the strong side to the weak side by Williams to allow for more sacks and big-play potential, with free agent Marcus Washington playing on the strong side. Barrow was signed to anchor the defense as the middle linebacker, and Gibbs said it is still possible the veteran could make his season debut Monday, although tendinitis has prevented Barrow from practicing regularly. He did not participate in all of the drills yesterday. "He continues to take more work," Gibbs said of Barrow.

Even if Barrow returns, he would have a supporting role -- Antonio Pierce (who is listed as probable for Monday night's game with a rib injury) has excelled as a middle linebacker in his place -- with Marshall expected to start in Arrington's spot. Marshall, who has not started since being signed from Tampa Bay as a free agent in 2001, has played primarily on special teams but worked on the weak and strong side in practices and preseason games. Marshall, an undrafted player from Michigan State, was inactive for four games last season but says he is ready.

"I know that my teammates feel comfortable with me going in," said Marshall, who has defended primarily on goal line and short packages this season. "And I feel comfortable going in and taking LaVar's place."

The Redskins have a versatile group of linebackers -- Pierce can play all three positions and Washington has experience playing inside or outside -- but suddenly lack depth, especially if Barrow is still out. Practice squad linebacker Clifton Smith is also out after elbow surgery.

Washington could sign a free agent to provide depth while Arrington is out and linebacker Steve Josue, who was signed this week, will likely be activated for Monday's game, with Khary Campbell the only other healthy linebacker on the roster. Linebacker Dominque Stevenson, who was cut last week, could also be resigned, Redskins officials said.

"Everybody has had a chance to take a lot of snaps at linebacker," Washington said, "and most guys can play more than one position, and I think that's just a plus for us. . . . I think we have guys who can come in and do the job."

There's no masking the significance of LaVar Arrington's injury. The linebacker is among several key players hurt entering Dallas game.The Redskins made a point of freeing up LaVar Arrington this season before a strained knee sidelined him.