When Mitch Kupchak had his damaged thumb repaired 1 1/2 months ago, the doctors apparently fixed more than torn ligaments. He thinks they also corrected some problems with his shooting touch.

"I wasn't shooting well just before I got hurt," Kupchak said yesterday, trying to keep a straight face. "So I told them to go in there during the operation and see what they could do to make me a better shooter."

Whatever they did has worked. Kupchak scored 21 points, the third game in a row he has totaled 20 or more, and Bob Dandridge had 26 as the Bullets romped to a 106-88 victory over the injury-riddled Houston Rockets despite 39 points by Calvin Murphy.

"A month ago, we probably wouldn't have won this game," said Bullet Coach Dick Motta, referring to the fact that Elvin Hayes had only nine points. "We didn't have anyone to take up the slack if 'E' or bobby Dandridge had an off game. But now Mitch is around. You've seen the difference."

the difference has been victories in five of the last six games since Kupchak's return. Of the 15 he missed, the Bullets won five.

After scoring seven points in the first two games. Kupchak has pumped in 13, 22, 22 and 21 the next four while making 26 of 44 shots. Just before the injury, he had missed about two-thirds of his last 41 shots.

"People say shooting is all mental," he said, "but at least with me, that's not true. If it was, I wouldn't be hitting now because my thumb would be on my mind.

"I'm really confident. Since the rest, I'm able to get open and hit the short jumper better than I was before I was hurt."

he also has benefited from a new way of taping the thumb.

"It was real tight the first few games, and it was restricting me," he said. "I've loosened it up and now I have more movement. I can feel how it helps."

So can the Bullets. They are a different club when Kupchak comes off the bench to dive after loose balls and sink his short jump shots. The only thing he isn't doing well is rebounding, but as has been the case lately, Hayes and Wes Unseld, with 15 each, didn't need much help yesterday in that area.

Washington overwhelmed the Rockets inside, outrebounding them by 22 and holding their front line to 10 points, 16 fewer than Dandridge scored.

"I thought we played as well as we can play right now," said Houston coach Tome Issalke, who is without both Rudy Tomjanovich and Moses Malone fractured foot). "They have a great inside attack and they worked us over. There really isn't much we can do about it."

The rockets led at the half, 48-47, after Murphy scored 22. But the Bullets held them to 14 points in the third quarter, a season-low for a Washington opponent, and spurted to a 74-62 advantage that grew as large as 22 points in the fourth period.

Dandridge did most of his damage early, getting 16 points in the first half working against rookies Robert Reid and Alonzo Bradley. In the third quarter. Motta purposely tried to get Kevin revey (22 points) more involved in the offense and Grevey responded with 10 points in 12 minutes.

"We called a few plays for Kevin," said Motta. "When he started to hit, things began to roll. But you have to feel sorry for Houston. We know what it's like to have their injury problems. They've got to rely on their outside shooting and it's hard to win that way."

At least yesterday, almost all of their shooting was done by Murphy who got off 30 shots, making 19. But neither of Houston's starting forwards scored until Dwight Jones put in a rebound midway through the third period.

Washington's victory, coupled with New York's loss to Boston, tied the Bullets and Knicks with 34-29 records as they battle for a favorable position in the first round of the playoffs. but Motta still is talking about catching San Antonio, which has a five-game lead over the Bullets in the Central Division.

"I'm curious about how we will do on San Antonio's home court," said Motta, looking ahead to the Bullets' game there Saturday. "I have a hunch we will be there later on (in the playoffs). We are playing pretty good right now and it will be a good test."

Phil Ghenier said yesterday his back was not much better. "You'd have to say it's doubtful I'll be back this season," he said. "It might take a miracle. . ." The Bullets don't play until Friday night in Houston. . . Hayes blocked six shots, running his two-game total to 17. He also made his only foul shot his 19th in a row.