The Houston Rockets changed their offense, changed their defense and threw out most of their usual caution today, then arose from the disarray the Celtics had left them in Saturday to defeat Boston, 91-86. The result: the best-of-seven National Basketball Association championship series is tied at two victories each.

Moses Malone again was a terror inside with 24 points and 22 rebounds, reborn with help on the boards from Robert Reid and in scoring by Mike Dunleavy. Dunleavy, self-admittedly one of the slowest and least exciting guards in the league, enters a phone booth now and then and reissues wearing a big S on his chest.

This was one such day.

The 6-foot-3 guard from South Carolina, signed by the Rockets as a free agent late in the 1977-78 season, scored 28 points, mostly on long jump shots and two-handed, scoop layups.

Thirteen of his points came in the third period as the Rockets broke away from a 50-50 halftime tie to a 75-67 lead. He and his teammates tired in the last quarter, but they never faltered.

"We made a lot of mistakes when we got behind and didn't even bend them (the Rockets) enough to see if they would break," said Celtic Coach Bill Fitch, who will get a chance to try again Tuesday night in Boston.

But this was destined to be the Rockets' day.

"We just put Saturday out of our minds and came out loose," Reid said, referring to Game 3's 94-71 humiliation.

The Rockets so controlled the rebounding that they got off 103 shots to 74 for the fast-breaking Celtics. And the home team's 28 offensive rebounds helped overcome its 36 percent shooting. Malone put up 30 shots, Dunleavy 22. Dunleavy had scored only 20 points in the three previous games of the series.

"Before, I was just going with the offense," he said. "I was looking for Moses all the time, no matter how much time was left on the 24-second clock. The Celtics made it clear to us yesterday (Saturday) that we couldn't always get the ball to Moses and that if we were going to win any more games, some other people were going to have to put the ball in."

Houston's Del Harris made a surprising coaching move by using only six players. The only Rocket reserve to play was forward Bill Willoughby, who relieved forwards Billy Paultz and Reid.

When Harris wanted to rest Dunleavy or the other guard, Tom Henderson, he put Reid in the back court.

"I decided after Saturday's game to go with six men," Harris said. "In going over the game with (General Manager) Ray Patterson, I just felt it was time. If we didn't win this game I was afraid the series would have been, for all intents and purposes, over. This was like the seventh game to us. We had to win.

"For the most part, substitutions just mess up the flow of the game anyway. You take a risk whenever you make one. Naturally, we'll make them for fatigue, injuries or lack of hustle, but we just didn't need to today.

"There's no poison here," he added. "It was a straight-up deal. I know people like (Allen) Leavell and (Calvin) Murphy didn't get to play, but I would have used them if the situation had dictated it."

Murphy was upset at not playing. "Winning is what counts and I've always said I'd do whatever it took for us to win," he said. "But no time at all, wow!"

Reid had 19 points and 10 offensive rebounds. Cedric Maxwell led Boston's scoring with 24 points while Robert Parish stayed out of foul trouble and had 18 points, 12 rebounds and five blocked shots. But Larry Bird, for the second straight game, had only eight points.

The Celtics didn't need his scoring Saturday, but they did today.

The Rockets came out smoking after intermission. They pounded the ball inside to Malone and, when the Celtics adjusted to that, took the outside shot or, surprise, ran fast breaks.

They outscored the Celtics, 10-2, in the first four minutes of the third perod, with Malone getting eight of the points. They also pulled down six offensive rebounds in that span, while the Celtics got only three shots, made only one.

With Dunleavy sinking the big shot when needed, Houston moved to a 10-point lead before the period ended. Its biggest leads was 11, in the final period.

The closest Boston came in the last 12 minutes was the five points it lost by, and the last points were scored with 70 seconds remaining, by Maxwell on a layup.