At about 9 o'clock this morning, John Riggins is expected to appear at what should be the last required hearing before an arbitrator issues a decision on his grievance case against the Redskins.

At 7:30 tonight, he will be across town at RFK Stadium, where he will make his first start since returning to pro football after a year's layoff.

Riggins' stormy relationship with the Redskins is just one reason why no one is quite sure what to expect from the team or from this opening preseason game against the Kansas City Chiefs (WMAL-630).

Joe Gibbs, the outgoing former San Diego offensive coordinator who was chosen to replace Jack Pardee and to jazz up Washington's offense, admits to a mild case of the jitters before his first game as a head coach.

"I'll probably be a little extra excited about this one," he said yesterday after a light morning workout at Carlisle, Pa. "But I really don't have any idea about how the game will go. I wouldn't mind having this game postponed a few weeks, because you always are sure you could have done something else to help yourself in preparations.

"But it will be good to find out what it is like to play on our home field in front of our home fans."

Riggens and Gibbs of course, won't be the only attractions tonight among the restructured Redskins, who have eight new starters. Spectators will need a program to identify Joe Washington, Terry Metcalf, Wilbur Young, Fred Cook and a bunch of promising draft choices.

It also will be interesting to see what kind of reception Riggens will receive from a crowd expected to number about 35,000 as he tries to return to the form that once brought-him recognition as one of the best backs in the NFL.

While sitting out last year in Kansas during a contract dispute, Riggens joked that he probably couldn't escape with his life if he tried to return to Washington, especially after many of the team's difficulties, which led to a 6-10 record, were blamed on his walkout. Yet his homespun philosophies and good ol' boy approach to living should make him one of the fans' favorites.

Not even Riggins' most severe critics have been able to complain about his attitude in training camp. He is in excellent condition, has picked up Gibbs' system quickly and has stayed after practices to run and work on his own.

"John is excited, I believe, about playing," Gibbs said. "It doesn't appear to me he's lost anything or has been hampered by the layoff. He's done everything we've asked of him. He's blocked well, he's smart and, man, what an effort he's given. I don't see any reason why he shouldn't be the same player he was before."

Riggins is a proud man who wasn't going to come back solely to have an impact on his arbitration case, in which he is seeking his $300,000 salary from last season.

Since his return, the Redskins and Riggins have maintained a quiet but steady truce, which is probably why both can live with a hearing and a game on the same day. It is expected that arbitrator Bert Luskin will issue a decision in October.

Riggins will start in the same backfield with Washington and quarterback Joe Theismann, who will play no more than a half, before being replaced by rookie Tom Flick. Gibbs said Mike Rae also could get in.

"We're going to use a lot of people," Gibbs said. "We have a lot of undecided spots and a lot of competition. Like at running back. hWe really don't have a first string. We are going to use maybe eight backs tonight to help us decide who will be doing the playing."

With rookie Russ Grimm nursing a let pull, Jerry Scanlan, who was a tackle until this week, will start at left guard over Ron Saul. Other offensive starters will be guard Melvin Jones, tackles, George Starke and Mark May, the No. 1 draft choice center Jeff Bostic, tight end Don Warren and receivers Art Monk and Ricky Thompson.

On defense, Gibbs, wil go with ends Coy Bacon and Karl Lorch, tackles Perry Brooks and Dave Bultz, linebackers Neal Olkewicz, Rich Milot and Monte Coleman, cornerbacks Lemar Parrish and Joe Lavender and safeties Tony Peters and Mark Murphy.

"Our priorities are first, player evaluation, and then after that, we'd like to win," Gibbs said. "I'd be kidding you if I said I didn't want to win. But it's important for us to see them in game conditions to see where we have advanced and what we need to work on."

Kansas City, like the Redskins, has a lot of young players. But the Chiefs, coming off an 8-8 season, are further in their rebuilding program.

The most interesting matchup should be the Chiefs' front defensive threesome, which features stars Art Still and Mike Bell, against the Redskins' young, unsettled offensive line. Washington also will get a chance to defend against quarterback Bill Kenney, who was released by the Redskins in 1978, and receiver-return man J. T. Smith, a Pro Bowler.

An arthroscopic examination of Mat Mendenhall's ailing knee was negative yesterday revealing no cartilage damage. He will return to camp and continue rehabilitation next week . . . center Bob Kuziel, who has a bad back, will not play tonight, nor will Brad Dusek, Ray Waddy and Larry Kubin, all recovering from injuries . . . Punters Mike Kirkland and Mike Connell each will have a half to kick . . . Mark Moseley probably will do the field-goal kicking, but not the kickoffs . . . Gibbs will use assistant coaches Don Breaux and Joe Bugel to signal the offensive plays. One is a decoy at all times . . . Gibbs said he would "stick to the basics because we haven't got everything in to our plans yet," so the Redskins offense may not be as wide open for the Chiefs as fans might hope . . . Pregame and halftime[WORDS ILLEGIBLE] be provided by the Almas and Kena Templ