The Washington Redskins finished a remarkable regular season yesterday by securing the No. 1 seed in next weekend's National Football Conference playoff tournament. But the celebration was muted by the sight of receiver Art Monk leaving the dressing room with a cast on his right foot.

Monk, one of Washington's most important players, further fractured his little toe in the opening minutes of the Redskins' methodical 28-0 victory over the St. Louis Cardinals at RFK Stadium. He will miss the playoffs, leaving Coach Joe Gibbs with a major offensive problem.

"I'm happy, but to be honest, the only thing I'm thinking about right now is how we can replace Art," said Gibbs, whose team will play its first-round playoff game against the Detroit Lions in RFK Saturday or Sunday.

By finishing 8-1, its best record in 40 years, Washington will be the home team for every NFC game it plays.

Gibbs said he was outraged that the No. 1 conference seed should not be guaranteed a Sunday game. He protested to the league last week but said, "I got no answer back. It's wrong. We earned some reward for being No. 1, and that should be a Sunday game. This way, we won't know our opponent until Tuesday morning and then we might have just four days to get ready."

The Redskins won despite the absence of Monk, fullback John Riggins (thigh bruise) and kick returner Mike Nelms (pulled thigh muscle). Halfback Joe Washington (sore knee) was in for limited action and cornerback Vernon Dean, who had an interception, incurred a fractured wrist during the game. Gibbs said he expects everyone but Monk to play this weekend.

Even Mark Moseley couldn't contribute to this victory. Moseley, whose field goals have been the point difference in six of the team's first seven wins, saw his field goal streak end at 23 over two seasons when he missed a 40-yarder in the first half. That now becomes the National Football League record. Moseley had made the first 20 attempts this season.

Yet Washington still handled the confused Cardinals thanks to another superb defensive performance. For the third straight week, the Redskins held an opponent to fewer than 200 yards. In the process, they got their first shutout since Oct. 19, 1980, thanks mainly to a pass rush that befuddled quarterback Neil Lomax and a run defense that limited Ottis Anderson to 31 yards.

The Cardinals gained only 18 yards in the first half, when they fell behind, 14-0. By the time the Redskins had a 21-0 lead, St. Louis had just 24 yards, Lomax had thrown two interceptions and the Cardinals had messed up a costly shotgun center snap.

"Against a young quarterback like Lomax, we mixed things up even more than usual," said free safety Mark Murphy. "We got them in a lot of second- and third-and-long situations. We knew we had to shut off Anderson, and if we did that, we would put ourselves in an advantageous situation."

Anderson, the only back to gain 100 yards on Washington this season (109 yards on Dec. 13), was neutralized early by an aggressive front four that dominated the St. Louis offensive line. It was the second-lowest rushing total of his four-year career. And the Cardinals, the NFC's leading rushing team, gained just 41 running yards.

Once Lomax had to pass, the Redskins blitzed regularly. Lomax couldn't handle the pressure, but Coach Jim Hanifan said he never seriously considered using veteran quarterback Jim Hart. Lomax was sacked five times, giving the Redskins 32 for the year, equaling their 1981 total in 16 games.

"We weren't doing anything offensively, but it wasn't Neil's fault," said Hanifan, whose team was held to 196 yards and had five turnovers. "The Redskins were more emotional than we were. They dominated the line. It was a bad day overall."

The defense also became the Redskins' best offense, causing turnovers and giving Washington good field position. Gibbs admitted the offense needed help while he and his staff tried to make up for the loss of Monk.

Monk, fourth in the NFC in receptions, hurt his foot in practice Thursday, then again Friday. X-rays showed what Gibbs called "a little hairline fracture that we thought he could play with." But on the Redskins' sixth offensive play, Monk felt the toe pop while making a cut. He finished the pattern and almost made a spectacular catch, then left the game.

With Monk out, Charlie Brown moved to his spot; Virgil Seay normally would have come in then, but he couldn't play because of a pulled hip muscle. That meant Alvin Garrett, who was listed as "definitely out" last Tuesday because of a sore ankle, took over. Monk's third-down man-in-motion responsibilities were shared by tight end Rick Walker and running back Nick Giaquinto.

"Things weren't too bad after a while," said Theismann, who dodged a good pass rush enough to complete 16 of 25 passes for 157 yards and three touchdowns. "I yelled a little but I think everyone will benefit from this next week."

(Gibbs said Brown and Seay, if healthy, will be the starting receivers against Detroit.)

Theismann's first scoring pass came with 4:46 left in the first quarter. It was set up when center Dan Dierdorf hiked the ball over Lomax's head on a third-down play from the Cardinal 38, with St. Louis in a shotgun formation. Defensive end Dexter Manley, later ejected for fighting with tackle Luis Sharpe, recovered at the 25.

On first down, St. Louis blitzed four linebackers but Theismann still managed to find Walker on a corner route into the end zone. Walker, who had beaten rookie Benny Perrin, caught the ball in full stride. Moseley's conversion made it 7-0.

Touchdown pass No. 2 came on the Redskins' ensuing possession. The two-yard throw to tight end Clint Didier ended a 52-yard drive and came after Theismann had scrambled eight yards to the two. It was Didier's first pro touchdown.

Moseley's streak ended near the end of the half. "I just missed it. I have no reason for it as far as I know, but I'll just have to start over," he said.

A pass by Lomax early in the third quarter that was batted by linebacker Mel Kaufman and intercepted by cornerback Jeris White at the Cardinal 25 led to a one-yard scoring run by Clarence Harmon and a 21-0 lead.

Harmon, usually used only in passing situations, got unexpected extra work yesterday. Riggins, who has had a thigh bruise for two weeks, decided in pregame warmups he couldn't play. And Joe Washington had enough discomfort in his sore right knee to reduce his playing time. So Harmon wound up with 21 carries for 63 yards, his second-best day as a pro.

The Cardinals' only real scoring threat came midway in the third period. They drove to the Redskin one, but Lomax pulled away from center before Dierdorf snapped the ball. Lomax fumbled and tackle Dave Butz recovered on the two.

Theismann then moved his team 98 yards, finishing the drive with his third touchdown pass, an eight-yarder to Joe Washington. The score was set up by a 46-yard pass interference call on cornerback Jeff Griffin against Brown. Griffin also was assessed an unnecessary roughness penalty on the play, in which he grabbed Brown around the neck and spun him to the ground. Brown had been trying to catch an option pass thrown by Washington.

Afterward, the Redskins had a subdued celebration, without any champagne. "We'll do our celebrating when we win the big one (Super Bowl)," Joe Washington said. "Our work is really just starting."