The Bullets' rebuilding plan suffered a frightening setback last night when Rick Mahorn took an awkward fall in the closing minutes of a 99-95 loss to Milwaukee at Capital Centre.

Mahorn, the team leader in rebounds and blocked shots and second in scoring, went over the top of the Bucks' 6-foot-11 Alton Lister and landed face first on the court.

The 6-10, 235-pounder's legs flipped over in a whiplash effect and he lay motionless on the court, on his side in a cramped position with his feet curled up. A stretcher was brought out and he was carried to an ambulance, which took him to the Bowie Clinic for X-rays and further examination.

The X-rays showed no fracture and the injury was diagnosed as a lower back strain. Mahorn will start heat treatments today and his status will be day to day. He is doubtful for Thursday night's game in Detroit and here Friday night against Houston.

"He said he landed on his face, but apparently broke his fall with his hands," said Carl MacCartee, the team physician. "It could have had a jamming effect on his lower back."

Coach Gene Shue said Jim Chones will start Thursday night, "but if Ricky going to be out for a while, we'll have to start looking around for another center.

"He was out of control when he fell," the coach said. "He's lucky he didn't break anything. It was an awkward fall. His hands hit first and his legs buckled. He said he felt he popped something in his back."

The Bullets' Jeff Ruland, who played his best game, said that Lister gave Mahorn a head fake. Mahorn lunged over him, like a person going off a diving board.

"It looked like he landed on his face, but he wasn't cut, so I guess he got his hands up in time," Ruland said. "But the way his legs came over made it so awkward."

Ruland, playing 31 minutes, scored 12 points and grabbed 10 rebounds, five offensive. The 6-11 rookie also blocked four shots and played adequate defense as the Bullets held their opponent under 100 points for the sixth straight game.

Milwaukee, playing without holdout Marques Johnson and injured Junior Bridgeman and Brian Winters, had their fourth successive victory secured when the Bullets rallied, scoring 21 of the game's final 24 points.

One reason for the Bullets' burst, which included three-point shots by Kevin Grevey and Greg Ballard, was that the Bucks had their reserves in the game.

"I took a big gamble," said Coach Don Nelson, when asked why he didn't put his regulars back in. "Some of the guys had been out so long, I was afraid they would be injury-prone if they went back in. I guess if we didn't have so many injuries, I wouldn't be thinking like that."

Bridgeman was put on the injured list Monday after re-straining a knee ligament. Winters tested his pulled groin muscle before the game and decided not to risk further injury.

With Quinn Buckner leading the way, the Bucks broke open a close game in the final four minutes of the first half. They scored 10 straight points to take a 53-45 lead and were ahead, 57-47, at halftime.

"I thought we played a terrific first half until the last few minutes," Shue said. "We were playing good defense until they went on that spree."

The Bucks quickly doubled their 10-point advantage in the third period. Sidney Moncrief started the half with two steals and Pat Cummings, in a rare starting role, blocked a shot, then scored five fast points.

Milwaukee was comfortably in front, 96-74, when the Bullets started their desperate full-court pressure defense. They forced the Bucks into seven turnovers in a four-minute stretch.

"I was pleased that we didn't give up," Shue said. "But the fact that it was close at the end was deceiving. Milwaukee had excellent control of the game until the very end."