Pat Fischer, the feisty little corner-back with the wicked wallop, was placed on the Washington Redskins' injured reserve list yesterday and will not play football again this season, if ever.

On a day when signs also were pointing to a switch in starting quarterbacks from Billy Kilmer to Joe Theismann, Redskin coach George Allen said he was told that Fisher would not be able to play again this year, although doctors have not been able to pinpoint the nature of the back injury. Allen said more tests were scheduled today.

A spokesperson in the office of Dr. Fred Luessenhop, the neurosurgeon treating Fischer, said last night no decision had been made whether to perform surgery.

Fischer, who had an operation after the 1971 season for a similar problem, told the spokesperson he preferred not to comment yesterday on his injury. Dr. Luessenhop also declined to comment.

Fischer has played 212 games at cornerback, more than any other player.

"It's a tough break for the Redskins and for Pat," Allen said yesterday after a two-hour workout that featured Theismann quarterbacking the first unit all day. "He's a great pro and a great person. I wish everyone had Pat's determination, his competitiveness and his love of the game. I just hope he's all right for the future."

Asked if he thought Fischer could ever play again, Allen said, "I would think it's doubtful. But knowing him, I wouldn't rule it out. After all, he's only 38.

"But you just don't replace a Pat Fischer. It's a real blow for us."

Nevertheless, Allen did add a new defensive back to his roster yesterday when he claimed five-year verteran Windlan Hall, a 27-year-old safety and cornerback off the Minnesota Viking waiver list.

Hall, 5-foot-11 and 175 pounds, was the captain of the Viking special teams this season before he was cut last Friday to make room for kicker Rick Danmeier because the Vikings had a surplus of defensive backs.

While Allen was shoring up his defense, several team sources told The Washington Post the coach also had decided to juice up his offense by giving Theismann his first start of the season.

Allen denied that report yesterday. He insisted Kilmer cut short his participation in practices yesterday because of his usual back problems. At least Allen now has a reasonable save-face excuse to offer for benching Kilmer after vigorously defending his quarterback Monday.

"We won't make a decision on any of these (injured) players before Thursday or Friday," Allen insisted when first asked about Kilmer's status.

When asked if he would start Kilmer if he is physically able to perform Sunday, Allen said:

"If he's feeling good, I think so. The reason we're losing is not Billy Kilmer. If everyone was playing as well as Billy Kilmer, we wouldn't be 3-3."

Kilmer seemed to have no difficulty throwing during warmups on the sidelines yesterday, and as the team ran through plays at the start of practice, Theismann was the quarterback for the first unit while Kilmer handled the second team.

During the offensive period of practice, Theismann also called a number of sprint-outs and roll-outs, passes far more suited to his style and strengths.

Kilmer, who did nothing but watch after the first 10 minutes of practice, declined to comment. Theismann also insisted that a switch "is news to me."

"It was just my typical Tuesday," Theismann said. "His back is hurting, so it's my turn to have a little fun. "That's all I know right now, honest."

Despite Allen's strong vote of confidence for Kilmer both yesterday and Monday, he has switched quarterbacks in the past under similar circumstances. During the 1973 season he said flatly on a Monday that Kilmer was his starter, only to open with Sonny Jurgensen the next Sunday.

One team source said Kilmer already has been told about the switch, and Allen spent a good portion of yesterday's practice talking with his veteran quarterback.

At the moment, the Redskins are averaging only 14 points a game and are ranked a lowly 10th in total offense in the NFC, 11th rushing, and 10th passing.

The Redskins were down to only three healthy running backs in practice yesterday, because Mike Thomas was in the trainer's room with a strained knee. He is expected to be available for the Eagle game, though Allen has said he plans to use Calvin Hill more often Sunday.

Defensive tackles Diron Talbert (hip pointer) and Bill Brundige (bruised knee) also missed the first workout of the week, but the Redskins are most concerned about the status of safety Jake Scott (pulled quadracep muscle) and receiver Charley Taylor (pulled jamstring).

Scott dressed for practive and showed up on the field, but spent all day standing around and is not expected to be available against the Eagles. Allen already has ruled Taylor out of action this week, and probably against the Colts Nov. 7.

Hall had an easy day - he showed up on the practice field just as the Redskins began their end-of-workout wind sprints . . . He was a fourth-round choice of the 49ers out of Arizona State in 1972 and spent four years in San Francisco.He was traded to the Vikings before the 1975 season . . . Eddie Brown will be the starting free safety in place of Scott and Danny Buggs will open at Taylor's wide receiver spot . . . Special teams coach Paul Lanham may give Brown a rest by having Larry Jones replace him on kickoff returns.