Defensive tackle Bob Slater's rookie season may have ended yesterday without the Washington Redskins' top draft pick having played a down.

Slater, on injured reserve so far this season because of a strained anterior cruciate ligament in his left knee, tried to elude offensive guard Bruce Kimball during practice yesterday. His left knee buckled and he was carried from the field.

Trainer Bubba Tyer later said that the initial diagnosis is torn cartilage. Team doctors will reevaluate the knee today and likely will perform arthroscopic surgery by early next week. Tyer said that such an injury could take six weeks to heal. Only six games remain in the regular season.

"The key now," Coach Joe Gibbs said resignedly, "is getting Bob ready for next year."

Slater had been practicing more than two weeks and appeared nearly ready to return to the active roster. It was not certain that the former University of Oklahoma standout would have been reactivated, however, even if he had been fully recovered.

The Redskins can reactivate only three more players from injured reserve during the regular season (plus one more in the postseason) and their injured list already includes all-pro receiver Charlie Brown, running back Joe Washington and all-pro free safety Mark Murphy. Furthermore, the Redskins' pass rush -- without Slater -- has 40 quarterback sacks, third-best in the league.

Slater, first hurt in the preseason finale at New Orleans Aug. 25, initially tried to practice more than a month ago. In his first practice, though, the pass-rushing specialist aggravated the strained ligament.

So he was told to rest two more weeks. "It was 100 percent-plus in strength when he came back again," Gibbs said. That was more than two weeks ago. And now this.

"To say the least," said Slater, 23, "it's been disappointing. I mean, at this point in time, it looks like there's no chance of me ever getting back this year.

"Before, the chance to play again is what kept this season from being a personal disaster. I always knew if somebody else on the defensive line got hurt, I'd be the next player to come in. Now, there's probably no doubt that they'll remove the cartilage.

"Doc (Stan) Lavine says it looks like it's the cartilage. He tapped his finger on the exact spot and when he did I was crawling up the wall (in pain)," Slater said. The 6-foot-4, 265 pounder said he hopes, at least, to to resume practicing before the season ends, trying to salvage something from his rookie season in the face of so much heartache.

When Slater was hurt yesterday, the starting defensive linemen and linebackers were nearby, fine-tuning pass-rushing assignments in a noncontact drill. When Slater fell, they stopped working; grim expressions and silence prevailed.

"We can only hope for the best," said defensive tackle Darryl Grant. "We'll help him out as much as he can. He's a teammate." And defensive end Todd Liebenstein, another player on injured reserve who has spent much locker room time this season talking with Slater, added, "We always talked about getting back on the roster. We tried not to talk about injuries. Those are behind us -- at least we thought they were."

Meanwhile, fullback John Riggins did not practice again yesterday and Gibbs said the chances are "50-50" that Riggins, 35, will play against Detroit Sunday afternoon at RFK Stadium. Riggins' lower back has been sore much of the season.

Gibbs reiterated that Riggins will make the final decision and that, if he is unable to play, rookie Keith Griffin will replace him.

"This is the most John's missed, the farthest off he's been (from practice) and the most plans we've made to play without him," Gibbs said.

The last time Riggins was listed as "50-50" was in the fourth week of the season. But he practiced on Thursday of that week, then rushed 140 yards on 33 carries in a 26-10 victory at New England.

"(Friday) is the crucial day," Gibbs said. "John could come to me (Friday) and say, 'Joe, this thing is sore.' Then, he would be out. Right now, I think this might be Keith's chance to step in and play."

Washington, who has been on the injured list for six weeks and is eligible to be reactivated, showed signs of recovery yesterday. He ran pass routes and practiced his favorite stutter-steps. Although he seemed impressive, he only returned to workouts two days ago and Gibbs has said that the running back will need at least two weeks of practice before he would be reactivated.

"I was kind of figuring Joe would come out and be real hesitant," Gibbs said. "But he was jumping around out there. It was a real positive sign."

"Things are going real well," said Washington, 31, who is considered vital to the Redskins, especially for his pass-catching abilities. "With John having to carry the ball so many times, I don't care if you're 13 years old, it will affect you."

Washington isn't sure when he'll be activated, although he is sure that he won't return at less than 100 percent. "When I come back, I come back," he said. "I don't come back to just bully-bully around."

He feels compassion for Slater, "a guy from Oklahoma, just like me," said Washington, who missed his entire rookie season of 1976 in San Diego because of knee injury. "I have to look after him."

Wide receiver Art Monk (lower back pains), special teams captain Greg Williams (bruised shoulder) and pass-rushing end Tony McGee (bruised knee) did not participate in contact drills, but are expected to play Sunday.

Quarterback Joe Theismann wore a black rubber glove on his left hand (bruised thumb, plus cuts and slight discoloration from getting kicked by Mark Moseley on a missed extra point Sunday). Theismann will not wear the glove in the game.

Just in case Theismann is unable to hold for Moseley's kicks Sunday, punter Jeff Hayes has been practicing the role. Theismann has held for Moseley since 1974.

Wide receiver Alvin Garrett (on injured reserve with an ankle injury) declined to comment on a $1.3 million suit filed against him by a Washington, D.C., man who charges that Garrett assaulted him in a District bar Jan. 14.