Redskin defensive right end Dennis Johnson's injured left knee has been placed in a cast for at least three weeks, coach George Allen said yesterday.

Johnson, injured in Sunday's 14-7 loss to Dallas, has a partially torn ligament. Allen said he will not place Johnson on the injured reserve list until team physician Stanford Lavine evaluates the injury later this week.

So Karl Lorch, who several coaches thought during training camp had a good shot at winning a starting position somewhere on the defensive line, finally will get to show what he can do as a full-time player Sunday at Buffalo.

The Redskins, struggling to stay in the race for a wild-card berth in the playoffs, have been beset by injuries and mistakes throughout a 6-5 season, with four defensive starters currently sidelined by major injuries.

"Karl is one of the brighter spots on the entire team." Allen said after a rare open practice. "Out of all our new, younger players. I would rate Karl the best prospect. I like his attitude. I like his dedication, and everytime we've used him, he's played well."

Ron McDole, the defensive left end, has watched Lorch develop for almost two seasons now and fill in whereever needed in addition to playing on the special teams, at one point with a dislocated elbow. Lorch is McDole's type of player.

"There's no doubt in my mind that Karl will eventually be All-Pro," McDole said. "He's the best prospect we've had on the defensive line since I've been here. He doesn't make the same mistake twice. You tell him something and you don't have to tell him again, which is the problem with a lot of young kids."

Actually, Lorch, 27, is a "young kid" only by Redskin standards. He is able to play all four down-linemen positions and is typical of the new breed of NEL linemen: strong and quick.

Lorch demonstrated that quickness soon after replacing Johnson Sunday. The Cowboys called a screen pass and Lorch quickly made it out to the flat to make the tackle and squelch what could have been a big play.

"It's great when you have a guy like Lorch," said McDole, "because, before, when somebody got hurt, I'd have to move inside or somebody else would have to move to a different position, so somebody else could play.

"Normally, you train a younger guy at only one position. That's bad enough in our system because it is a very complex system, and learning all the positions at one time is very difficult. So you teach them one position, until they get confident at that. Well, Karl's always been an exception to that."

Lorch, a native Hawaiian, said he prefers right and, because it is a position he played at Southern California and with the Hawaii team of the defunct World Football League.

But, he added, the front-four positions are "not really that different. They're basically the same, except you're blocked more on the inside."

Allen said he thinks Lorch's future is at end, and that Lorch is the likely heir to McDole whenever the 38-year-old veteran retires. Lorch played end much of the exhibition season, with the praise he got leading to speculation he would open the season at Johnson's position.

yesterday, Lorch said he was happy it did not work out that way.

"It takes time to just work in," he said. "It was better for me to go gradually because it's so hard to learn these defenses, especially the audibles when you're playing different positions. You have to build up your confidence. I'm comfortable now."

Lorch was another player Allen obtained for a draft choice. In contrast to some deals made for high draft choices that have not panned out, the trade for Lorch appears to be a coup.

Miami originally took Lorch in the ninth round of the 1973 draft. But the signed with the Hawaiians for the 1974 season.The Redskins extensively scouted the WFL and Allen liked what he saw in Lorch. On Jan. 30, 1975, the Redskins traded their 10th-round pick in 1976 for the negoiating rights to Lorch.

Allen said yesterday Lorch had agreed to play for Washington that season, but backed out at last moment.

"I just wanted to help Hawaii have a chance at having a pro football team," Lorch said.

Lorch was one of the lucky ones in the WFL's final season. He did not miss a paycheck before the league folded after the 11th game of the season.

"He's worth a first or second-round choice right now," Allen said. "You couldn't draft a guy like that right now."

At least, he couldn't. The Redskins' first pick in the next draft is in the ninth round.

If Allen is going to change quarter backs this week, it wasn't evident yesterday. Joe Theismann worked most of the offensive drill and Billy Kilmer emulated Bill's quarterback Joe Ferguson against the Redskin defense . . . Starting fullback Clarence Harmon was recovered enough from a leg injury suffered against Dallas to run with the first string . . . Bob Kuziel started at center in place of Len Hauss, who has a broken rib but should play Sunday . . . Jerry Smith was running at tight end in place of Jean Fugett, who was excused early to get treatment for some bumps and bruises . . . Defensive tackle Bill Brundige dressed for his first practice in a week, but couldn't walk well because of his sprained left foot . . . Line-backer Chris Hanburger was still limping noticeably on his reinjured left knee . . . Safety Jake Scott was not on the field, but he wasn't all last week either and his several broken ribs did not keep him from playing . . . McDole played on the Bill's two championship teams in the AFL and has lived in Buffalo. Asked to give his advice on how teammates could keep warm Sunday.McDole said, "Wrap warmly." But he said he does not expect cold weather to be a factor "unless it's bitter, bitter cold and when you spit, it almost freezes."