It was a game that clearly demonstrated everything that was wrong with the Washington Redskins' defense last season.

Nov. 8, RFK Stadium. The opponent: the Detroit Lions. The Redskins win, 33-31, but not until the Lions have gained 499 yards and not until running back Billy Sims has gained 159.

Sims had the two longest gains of the season against the Redskins, 51 and 29 yards. Six times, Lions runners carried at least 13 yards, working mainly on the flanks.

Now, a year later, the Redskins' playoff hopes against Detroit and Sims Saturday in RFK rest heavily on that defensive unit, a unit that finally has been able to give Dave Butz some help stopping the run.

While the offense struggles with injuries, the defense is getting stronger every week. In the last three games, no team has gained more than 200 yards against the Redskins. For the season, no team in the NFL gave up fewer points.

"Thank goodness," quarterback Joe Theismann said, "that the defense's timing is so good. What's that saying, that the teams who do well in the playoffs are the ones who play best defensively?"

Statistically, this is the Redskins' best defense since that of George Allen's 1974 team, which gave up only 234 yards a game, 102.8 on the ground and 131 passing. The 1982 Redskins' numbers are 296, 105.1 and 190.4.

But consider the 1981 numbers: 325.4, 135.1 and 190.3. Obviously, rushing defense was the major reason the team gave up almost 22 points a game. And no one illustrated that better than Sims 14 months ago.

Now the defense, with six new starters since the beginning of last season, is yielding only 14 points a game, and only 11.7 since the 34-point opener against Philadelphia. And the major reason for the improvement is the Redskins' increased boldness stopping the run. Even when tackle Perry Brooks was sidelined with a knee injury and inexperienced Darryl Grant took his place, the unit didn't falter.

It is no coincidence that this has been Butz's best season in his seven years with the Redskins. He no longer is inconsistent. Instead, he clogs up the middle with his 295 pounds, inspiring teammates with occasional shocking tackles and quarterback sacks.

"Dave doesn't say much but we all get lifted when he makes one of those tackles where the back goes flying like he hit against a brick wall," said safety Mark Murphy, the team's leading tackler for the fourth straight year.

"Dave is the one dominant player we have in the front line," Murphy said. "Offenses have to use two or three guys sometimes to handle him and that opens it up for everyone else."

Opponents knew how to avoid Butz last year. Just go around either end. That's one reason, in the final nine games of 1981, the Redskins gave up 38 rushing plays of at least 10 yards, seven of at least 20. They yielded six 100-yard-plus games by individual backs.

But in this nine-game season, opponents have only 18 gains of 10 yards or better, including five of at least 20 yards. And St. Louis back Ottis Anderson remains the only runner to gain 100 yards against the Redskins this season.

This year, when teams try to go outside, the defense is better prepared.

Ends Dexter Manley and Mat Mendenhall, both first-time starters last season, have matured rapidly. Mendenhall's strength always has been against the run, but Manley no longer falls victim frequently to sucker blocks and trap plays.

Right linebacker Rich Milot, who struggled stopping the rush last year, is the team's No. 3 tackler (behind Murphy and middle linebacker Neal Olkewicz) because of his new-found aggressiveness against running backs. Left linebacker Mel Kaufman, filling in for injured Monte Coleman, has been surprisingly strong, considering he was an inexperienced rookie free agent last season. Pro Bowler Tony Peters and Murphy both became more active on running plays, with Murphy willing to come up faster to the line of scrimmage to make his tackles.

And new cornerbacks Jeris White and Vernon Dean have contributed greatly by forcing the run more quickly and tackling with authority.

That's one reason Coach Joe Gibbs was so concerned yesterday when Dean's knee locked up early in practice. Dean went to the training room and didn't return, but both he and Gibbs said later that he should be all right today.

"I'm fine," said Dean, one of the league's finest defensive rookies. "I stepped back (in practice) and it locked on me. But after getting a treatment, it feels fine, just a little sore. I'll be able to play; no problem."

Torgy Torgeson has been the Redskins' defensive line coach since the Allen years. He says this team's improvement against the run can be traced to "the fact we've meshed as a group. We tried to . . . make things happen against the run right from the start of training camp but, realistically, we probably are ahead of where I thought we'd be . . .

"The secondary has done a great job of coming up and supporting and that's helped. . . . It's been catching. They've played at a high level every week and now they are convinced they can do the job.

"And the nice part is, they still are basically young. They should get better and better."

Halfback Clarence Harmon, plagued by knee and hand injuries (cracked bone), probably won't play Saturday, Gibbs said . . . Fullback John Riggins again had a full practice, but Joe Washington saw limited work. Gibbs expects both to play against Detroit.