Arthroscopic surgery performed on Redskins running back Joe Washington revealed yesterday that the cartilage in his left knee suffered a larger tear than previously thought. That could prolong his recovery beyond the original projection of three to six weeks.

The operation, performed by Dr. Stanford Lavine at Sibley Hospital, also uncovered some slight, but unexpected fraying of the cruciate ligament. But the Redskins are more concerned about the extent of the cartilage damage.

Coach Joe Gibbs said yesterday he still expects Washington to return to the lineup in five or six weeks.

But it was learned that if the cartilage does not heal in about four weeks as anticipated by Lavine, there is a possibility Washington will need more surgery.

However, Gibbs said he had every reason to believe "that the knee will heal within the time they say. Anytime you have an operation like this, I guess there's always a chance it may not be enough. But I have no reason to think that everything won't go all right."

Washington will be included on the Redskins' final 49-man roster, Gibbs said, and then a decision will be made whether to place him on the in-season injured reserve list. If put on that list, he would have to sit out at least four weeks of the regular season before he can be automatically activated without going through waivers.

Lavine, reached at his office yesterday afternoon, would not comment further on the operation. But Gibbs said, "They took out a small part of his cartilage that was broken off. It wasn't a situation where it was a piece, but part of the cartilage was broken off." The official medical report said it was a "partial removal of the lateral meniscus cartilage."

Said Gibbs: "Now, it's just a matter of how long it takes him to come back. If I had to guess, it would be five to six weeks. But that's only a guess. Maybe it will be a bit faster than that if he is feeling good."

Washington, the Redskins' most productive running back last season, injured the knee making a cut on the RFK Stadium grass in the first quarter of the team's exhibition loss to Buffalo on Friday.

Losing Washington for this length of time is cause for major concern to the Redskins. Despite missing much or all of five games last year, Washington led the team with 916 yards rushing and caught 70 passes. In the games Washington did not play, the Redskins were 0-5. There also is no one on the current roster who can replace his quickness.

Gibbs said he had a long talk with veteran fullback John Riggins yesterday and told him to expect more responsibilities as long as Washington is out.

"John Riggins is gonna have to carry a big load. We're going to lean heavily on him," Gibbs said after practice. "We'll play Otis Wonsley and Wilbur Jackson a lot this weekend (in the final exhibition game at Cincinnati). It's a matter of who's gonna come in behind John to do the job for us."