The Bullets will open their NBA Eastern Conference championship showdown with San Antonio tonight not knowing how much supersub Mitch Kupchak will be able to contribute during the series.

Kupchak, who has been fighting soreness in his back since mid-March, missed another practice yesterday because of the pain.

Coach Dick Motta said he is not counting on Kupchak for the 8:35 p.m. contest before a sellout crowd in Capital Centre (WDCA-TV-20 on delayed basis beginning at 10 p.m.)

Kupchak is scheduled to meet with team physician Stanford Lavine before the game to determine future treatments.

"When Mitch misses practices, you know his back is really huring him," Motta said. "I know it is getting to him mentally, and you feel for him."

Although Kupchak had only limited success against Atlanta in the semifinal round, the Bullets could use his offensive abilities against the Spurs. He functions best in a quick tempo, the kind expected in this round. He also could be used by Motta against Spur center Mike Green, who is more mobile than the other San Antonio pivot man, Bill Paultz.

Tempo will be the key word in this series replacing the "intensity" of the Atlanta confrontation. The Spurs want to run at all times; the Bullets want to pick their spots, controlling the pace and trying to force San Antonio into a half-court game.

"This should be a more comfortable series for us," said forward Bob Dandridge. "We should be able to run instead of having to set it up every time like we did against Atlanta.

"Just like last year, we want to make sure we don't get into a shootout with them. But I think we have the type of team that can adjust it up."

Unlike Atlanta, San Antonio gears its game around its offense, not its defense. Although Coach Doug Moe claims his Spurs play better at the defensive end than anyone gives them credit-including the referees-they still aren't the same type of tenacious, scratch-claw fantatics produced by Atlanta Coach Hubie Brown.

In last year's conference finals, which Washington won in six games despite San Antonio's owning a homecourt advantage, the Bullets had little trouble executing their standard offense.

The Spures were able to stay in contention only when they shot exceptionally well. Otherwise, they were beaten at their own running game so decisively that, by late in the series they went to a set-it-up strategy out of desperation.

"I hope to come out of this series with more fast breaks than they have," Motta said. "If we execute properly against them, we will get the shots we want. We just don't want to run and gun with them like Philly did."

If San Antonio, which is coming off a rugged seven-game series against the 76ers, is to win this round, the Spurs will have to get consistently high production from three players: league-leading scorer George Gervin, fellow guard James Silas and forward Larry Kenon. And the supporting cast will have to contribute more than it did in most games last year against the Bullets.

Gervin, who averaged 29 points in the regular season, has had little problem in the past equaling or surpassing that total against Washington. The Bullets, however, want to make sure he works for his baskets and is forced to play defense.

Tom Henderson, who prefers to cover shooting guards instead of ball-handlers, most likely will start on Gervin. In last year's sixth game, he did a fine job in the second half on the Iceman, providing an edge that led to Washington's victory.

"Gervin is going to get his points," Henderson said. "There is no way to stop him. But we just don't want to give him a lot of easy baskets. The key is to deny him the ball, and when he gets it, make him work for a shot."

Silas, once a superstar in the old American Basketball Association regained a starting spot this season after struggling with knee problems since San Antonio joined the NBA. He averaged 21.8 points in four games against Washington this year, mostly off his specialty: a back-in, turnaround jumper from the top of the key.

"That's my man, James Silas," Gervin said. "He wasn't around for them last year, but he's here now. And he will make a difference."

But the Spurs have learned the hard way they can't beat Washington with scoring only from the back court. So Kenon, the 6-foot-9 man of a million moves, will have to produce up front, probably aginst the guarding of Dandridge, at least in some of the series.

"You make him play defense," said Dandridge, who is 6-6. "You turn it into a full-court game for him and not just let him concentrate on offense. He's a fine offensive player, especially if he gets loose. The one thing we don't want is for him to go up and down the court with the ball."

"I think this will be a more rugged series than last year," Bullet center Wes Unseld said. "They will be much more determined because they want to make up for us beating them."

Paultz, one of the league's best centre in Unseld's estimation, has been hampered by a hamstring pull but is expected to play tonight. Green, who cavorts much more like a forward, will share time with him in the pivot, although he gives away about 50 pounds to Unseld.

As in any game with the Bullets, rebounding is an important facet. The Spurs almost equaled Washington's rebounding total in last year's series but weren't able to hold their own on the boards in close games.

"They (the Spurs) are a bunch of free spirits," Motta said. "That's not meant as a knock, but it makes them want to play. You never know what they are going to do." CAPTION: Picture, Wes Unseld passes over Charles Johnson to Tom Henderson (14) in Bullet workout at Capital Centre yesterday.By Richard Darcey-The Washington Post