The loss of Rich Caster, probably for the season, with damaged knee cartilage could force the Redskins to modify substantially their use of a two-tight end offense, despite its success the past three weeks.

"We are going to have to sit down and think about what we are going to do," Coach Joe Gibbs said yesterday. "The way Rich was playing, he looked like just the guy we wanted for our offense. We might have to go back to using two running backs again."

Caster, signed by the Redskins off the waiver list three weeks ago, will undergo surgery on his left knee today and will miss at least four weeks. But Gibbs admitted that, with only eight weeks remaining in the season, it could be difficult for Caster to return.

Since committing themselves to a two-tight end, one-running back alignment, the Redskins have won two of their last three games. During that time, they have reduced their turnovers and penalties.

But to make the offense work, Gibbs needs a pass-catching tight end, a role Caster was beginning to fill. Until Caster got hurt in the first quarter of Sunday's 24-22 victory over New England, the Redskins had planned to use much of the same offense San Diego employed last season to free tight end Kellen Winslow, with Caster filling Winslow's role.

The Redskins' remaining tight ends are Don Warren, who is a fine run blocker, but does not have Caster's quickness on pass routes, and Rick Walker, who is faster than Warren, but has caught only eight passes and is slowed by a sore hip.

Gibbs now is left searching for a tight end who can be an effective receiver. With the trading deadline past, the Redskins will have to settle for a free agent. Gibbs yesterday said one possibility is Bob Raba, the ex-Maryland player cut in training camp. But Raba hardly fits the Caster mold.

"At this late time, it's hard to get another player who has Rich's abilities," Gibbs said. "He was a steal, really. You hate to give up on the two tight ends, because that's what I'm comfortable with. It's like going back to square one again, but we've been doing that all season, anyway."

Because of the success the Redskins have had running the ball with two tight ends, Gibbs seems certain to maintain that alignment. But to pass effectively, he also may have to use John Riggins and Joe Washington in the same backfield, with Washington going in motion as a wide receiver.

Caster's injury wasn't Gibbs' only worry yesterday. He was concerned with the performance of his pass defense, which has given up 602 yards the past two weeks.

Despite these back-to-back mediocre performances, Defensive Coordinator Richie Petitbon predicted yesterday that the Redskin secondary would lead the league in pass defense again by season's end.

"When this is all finished, we will be No. 1," Petitbon said. "I'll tell you this, there are a lot of people who would swap personnel with us. We aren't playing that well right now, no question about it. Our pass rush has to be better and we aren't covering as well as we are capable. But it will come around."

Petitbon bases some of his optimism on the assumption that cornerback Lemar Parrish will improve soon. Hindered by injuries since training camp, Parrish doesn't have an interception, nor is his pass defense up to his usual standards.

"Lemar isn't happy with how he is playing and we aren't either," Petitbon said. "But he's finally pretty healthy. He's getting more work in practice and that has to help him. He still can play, I'm convinced of that. But he has to do well, because he means to much to us."

The Redskins have been particularly vulnerable to long passes the last two weeks. Part of the reason is that Miami and New England have good passing games and receivers. Petitbon says another factor is the Redskin injuries, which are forcing him "to patch and plug every week. We haven't had the same healthy unit all season, and people we are playing with now aren't all healthy, either."

Safety Tony Peters says lack of a consistent pass rush also is hurting the coverage. "Right now, we have a weak pass rush," he said. "I don't care who is doing the covering, you are going to give up long passes if you don't get some pressure on the passer."

The Redskins, who have only 15 sacks in eight games, began the season knowing they had a pass-rushing problem. Then they waived their best rusher, Coy Bacon, because of disciplinary troubles. And tackle Wilbur Young, who was acquired from San Diego to bolster the line, has been, in Petitbon's words, "a very big disappointment. He just hasn't delivered."

Young, who has been starting since Perry Brooks suffered a broken thumb, doesn't have a sack. Petitbon said Brooks will replace Young against St. Louis Sunday.

The team's best pass rusher now is rookie Dexter Manley, who doesn't start. Petitbon said Dallas Hickman, re-signed two weeks ago, could play a more prominent rushing role.

Washington had a chance to alleviate this front four situation by acquiring Fred Dean from San Diego three weeks ago. But sources said the team entered the bidding just as the Chargers and San Francisco had reached an agreement. The 49ers gave up a second-round choice for Dean.

Gibbs said running back Otis Wonsley (sprained ankle) probably would miss the Cardinal game, but he expected lineman Joe Jacoby to be available.