RFK Stadium probably has never been as quiet during a Washington Redskins game as it was at 10 o'clock last night when the public address announcer said quarterback Joe Theismann, who had started the last 71 games for the Redskins, had suffered a compound fracture of his lower right leg.

At 1:30 this morning at Arlington Hospital, the team's orthopedic surgeon, Dr. Charles Jackson, made an even more chilling announcement after 40 minutes of surgery: Theismann's right leg will be in a cast for three months, meaning that his season is over.

Perhaps equally stunning was what happened at RFK. Jay Schroeder, a second-year pro who had thrown only eight passes all season, came on to complete 13 of 20 for 221 yards and one touchdown, a 14-yard game-winner to Clint Didier, as the Redskins beat the New York Giants, 23-21. Details on Page D1.

The victory moved the Redskins one game behind the Giants and Dallas Cowboys in the National Football Conference's East Division.

The severity of the injury raised immediate speculation that the career of Theismann, at 36 the oldest starting quarterback in the National Football League, may be in jeopardy.

But Jackson said, "The prognosis right now is a very good one . . . I expect he will be an excellent football player again next year."

Jackson said Theismann suffered "an open fracture of the tibia (shinbone) and the fibula (the long, thin outer bone running between the knee and ankle), with broken skin." A compound fracture means that the broken edges of a bone are pushed through the skin.

A hospital spokesman said that Theismann would remain in the hospital for at least 10 days.

Jackson began performing surgery just before midnight to clean the wound, put in a pack and several sutures, then place the leg in a cast. "The X-rays look very good and Joe's doing real well," Jackson said afterward. "He's a very motivated, confident and courageous human being."

Perhaps Theismann's most remarkable trait has been his durability. Only once since 1978, when he became entrenched as the Redskins' starting quarterback, has he been unable to play his position, and that was in a 1980 game, when he still managed to hold for field goal and extra-point attempts. He had participated in 163 in a row.

But after a trick play in the first minute of the second quarter, called a flea-flicker, resulted in his being sacked by three Giants, including all-pro linebacker Lawrence Taylor, Theismann was left sprawled on the bermuda grass of RFK, his lower leg pinned under his body.

Theismann had just taken a pitchout from running back John Riggins when he was hit by Taylor. Another New York linebacker, Gary Reasons, came over the top, and Theismann's right ankle was twisted underneath him.

Taylor immediately beckoned for Redskins doctors, who rushed onto the field, followed shortly by Coach Joe Gibbs. Several Giants stood by Theismann as a stretcher was rolled onto the field.

Among the Giants on the field was linebacker Harry Carson, who was the first to make contact with Theismann on the play, although he did not fall on him. "I saw that his leg was broken in two," Carson said. "My heart goes out to him."

After several minutes Theismann was removed, and the usual sellout crowd of more than 55,000 cheered nervously. Club owner Jack Kent Cooke paced in his box.

An ambulance brought Theismann to Arlington Hospital, where he was joined by his fiance, actress Cathy Lee Crosby.

Just after midnight, as doctors performed surgery on Theismann in a nearby operating room, a group gathered around a TV set in the lobby and cheered as cornerback Vernon Dean intercepted a Giants pass late in the game to ensure a Redskins victory.

"This game," said one, "is for Joe."

Jackson said Theismann watched about 10 minutes of the game before the operation, without having taken an anesthetic.

"He has a very high threshold of pain," the doctor said.

Ever since the Redskins began their ascent toward the Super Bowl in 1982, there has been the constant fear: What if Joe Theismann gets hurt?

Now, faced with exactly that situation, the team's quarterback coach, Jerry Rhome, said that Babe Laufenberg would be brought back this week to back up Schroeder. Laufenberg was released near the end of training camp.

Jackson was asked how long Theismann could be out of action, and said: "It could be up to as long as a year, but I don't think so. I'm optimistic."

Theismann came out of Notre Dame in 1971, drafted in the fourth round by Miami. He played three years in Canada before he joined the Redskins, who had obtained his rights in 1974 from the Dolphins. He became the permanent Washington starter in 1978 when Jack Pardee took over for George Allen as head coach.

He is the most prolific passer in Redskins history. He started the 1985 season with 23,432 yards passing, 1,877 completions and 3,301 attempts. His career rating of 79.0 makes him 11th on the all-time NFL passing list behind Bart Starr with 80.3. But he entered last night's game ranking as the second-lowest rated quarterback in the NFC this season.

For one night at least, Schroeder was a magnificent replacement. The score at the time of Theismann's injury was 7-7, and Theismann had completed seven passes in 10 attempts for 50 yards and one touchdown. He had been sacked three times.

Thanks to Schroeder's heroics, the Redskins are 6-5 with five weeks remaining in the regular season. It's a period of time in which Schroeder will be in the spotlight, and Theismann will be in a cast.