The Washington Bullets' hopes of winning the NBA Eastern Conference championship were jolted yesterday when Wes Unseld was sidelined for at least one game and probably two with a badly sprained ankle.

Unseld, who hurt the ankle in the final minute of regulation in Sunday's overtime Bullet victory in the series opener at Philadelphia, will be in a cast at least until tomorrow night, when the injury will be re-examined by team physician Dr. Stanford Lavine.

Lavine took a X-ray of the ankle yesterday and determined there was no break.

"We were afraid something was broken," said Coach Dick Motta, "so at least it wasn't all bad news. I'm not going to make any predictions that he won't be back Friday night. Wes is a quick healer, so you never can say how fast he'll get over an injury."

Unseld will not suit up for tomorrow's second game in Philadelphia and Lavine said he was "questionable" for game three Friday night in Capital Centre. Mitch Kupchak will take his place in the starting lineup.

The Bullets feel they especially need Unseld back for Friday. They figure they can capture this best-of-seven series by winning the three scheduled games in the Centre, but without Unseld's rebounding and outlet passing, that will be a difficult task.

"You think you have the injuires behind you and now this," said Motta, who has endured a rash of injuries the last half of the season. "It was such a freak play. That really makes it hurt."

Unseld was hurt while scrambling with Philadelphia guard Lloyd Free for a loose ball. Unseld said that "our feet tangled and I turned my ankle."

He played 48 more seconds of the fourth period and then a minute of overtime before pulling himself out. Lavine placed the ankle in a cast immediately after the game.

Unseld has been outstanding in the playoffs, averaging 37 minutes, 11 points and 10 rebounds. Just as importantly, he was setting many picks in the Bullet offense to free teammates for open shots and layups. And he was second behind guard Tom Henderson in assists.

Unseld had 10 of Washington's 64 rebounds Sunday as the Bullets controlled the boards and prevented Philadelphia from working its fast break. The 76ers also had little success driving the middle for layups off their set offense.

"If we don't rebound, they are going to run on us," said Motta. "We can't let them run wild and expect to win. If we rebound, we can keep control of the tempo. If we don't, it's going to be tough."

Although Kupchak had 14 rebounds in the opener, he has not been as consistent on the boards as Unseld. And Kupchak admists he does not set the same kind of crushing picks.

"I'm not going to try to play like Wes," he said."I can't do that. I can be there and set the pick but it won't have the same effect. I can also throw an outlet pass to half-court but not to three-quarters court like he can.

"I'll try to be more conscious of setting picks and freeing people for open baseline shots, but I have to play in my style. The offense in flexible enough to accommodate what I do best, much more so than last year."

What Kupchak does best is run and score. He has been struggling during the playoffs to get his shooting on target (44 percent compared to 52 percent for the season), but he says knowing he will start will help him.

"It's comforting going into a game knowing what your duties are," he said. "I can prepare myself better for the game. I can prepare for who I will guard and where I will play.

"That's been part of my problem lately. My role has been changing so much. I played a lot of minutes at the end of the season for Bobby (Dandridge) when I felt I had to score and then I started two playoff games for him.

"I felt I had to be more mechanical in his spot (small forward) in the playoffs and pass and run the plays. But I play better when I let things happen. I have to be free and loose."

Kupchak, who has played well against the 76ers this season, was more relaxed and agressive Sunday. He said he could have shot better (five for 14), "but I liked the emmpo. Itwas fast and open, which helps me." He has averaged only 10 points in nine playoff games.

With Kupchak in the lineup, Motta feels Philadelphia will have to be "more concious of our offense than with Wes. They can't hide (George) McGinnis on Mitch." McGinnis, the 76ers' weakest defensive player, had been guarding Unseld while center Caldwell Jones switched to Elvin Haves.

Motta said that seldom-used Joe Pace would back up Kupchak - "Joe plays well against Philly, too" - and that Hayes also would play some enter. Rookie forward Greg Ballard will get Kupchak's minutes at the forward positions.

"I still feel good about this team," said Motta. "We are playing very good and we are moving the ball well."

The Bullets watched films of the opener yesterday but did not have a formal practice. Kupchak, Ballard and Kenvin Grevey stayed late and worked on their shooting . . . The Philadelphia coaches are complaining about the Bullets' physical play, charging that the officials let Unseld and Hayes push around their players. Tomorrow night's game will be televised on WDCA-TV-20at 8 o'clock.