Perry Brooks, the hulking young defensive tackle bidding for a starting position with the Washington Redskins, threw a large scare into the coaching staff yesterday when he suffered what is now being described as a mild sprain to the medial ligament of the right knee.

The injury occurred four plays from the end of the week's final practice and will keep Brooks out of today's rookie scrimmage against the Colts at Annapolis at 2 p.m.

Nevertheless, Coach Jack Pardee was relieved when he went into the training room and learned that Brooks' injury was not as serious as first thought. Brooks had lain writhing in pain on the field clutching his knee and was carried off by two trainers.

"It doesn't look very bad at this point," Pardee said. "If it doesn't hemorrhage too much, he ought to be starting to work on the weights in two or three days. If it's just a mild sprain, he'll have to miss a few days. But you never know about that."

Brooks will remain in Carlisle today to undergo treatment and will be examined tomorrow by team physician Stanford Lavine.

Brooks, 23, a 6-foot-3, 260-pounder from Southern University, spent the 1977 season on the club's injured reserve list after damaging ligaments in his left knee in a preseason game against the Miami Dolphins.

With Bill Brundige out for the season because of foot surgery last week, Brooks is being counted on heavily to help provide a pass rush. Pardee said yesterday he plans to substitute freely on his defensive line this year, and Brooks probably would see extensive action.

Brooks was hurt on a sweep during an 11-on-11 drill when he was blocked by veteran center Ted Fritsch.

"It was a clean block," Fritsch said. "I feel bad about it because Perry's my buddy. I thought it was a pretty good block myself. But it's just like anything else. You can walk across the street and get hit by a car, too. It's part of the game."

Pardee said it appeared that Brooks "jumped up" and was hit, and that the jump probably prevented a more serious injury. "It's not as if the knee was planted and he got hit," Pardee said.

"He's had a good week, and we're counting on him. We need to get him well and back in action and get him some experience. This is a valuable time for him."

Earlier in the day, General Manager Bobby Beathard had described Brooks as "one of the best things to come out of the week.

"He's the type of defensive lineman you look for all the time when you're scouting. He's got the size and the ability to rush the passer. He thoroughly enjoys the game and I like to see those kind of guys. He has the potential to be a starter for us."

The Redskins had hoped to get a long look at Brooks in today's scrimmage, but they will have plenty of other prospects to examine.

In addition to Brooks, Beathard said he has been impressed with wide receiver Brian Fryer, who has been out-classing defensive backs all week with his fine moves and speed.

Fryer has missed the last two seasons with knee problems, and still must demonstrate that he is durable enough to play regularly. He under-went knee surgery a year ago and has not had any contact since.

"We think he can play," said Beathard. "He's looked great, and he seems to be getting better all the time. He's got great concentration and he's a clever receiver, very difficult to cover. He's got a great shot at playing, too."

The sleeper of Pardee's first Redskin training camp as coach probably will start at fullback against the Colts. His name is Nate Jackson, and his game is raw power. Jackson is 6-1 and weights 240 pounds. "I'll get him down to 235," said George Dickson, the backfield coach. "He could be a good one."

Jackson, once an 11th-round draft choice by Buffalo, failed in a tryout with Montreal earlier this summer. "A guy with that kind of size and that often," said Beathard. "If he gets some of that weight off, he's got a heck of a shot."

A number of other newcomers have made strong impressions and should see plenty of action today. They include wide receivers Walker Lee and Larry Franklin; offensive guard Jim Harlan; linebackers Don Hover and Chris Tormey and defensive backs Mike Williams and J. T. Smith.

The major disappointment of the week was rookie running back Tony Green, who pulled up lame with a hamstring problem Wednesday and has not practiced since. The Redskins' first draft pick (in the sixth round) will not play today, and Beathard plainly is not pleased about it.

Neither the Colts nor the Redskins will play their front-line quarterbacks. Pardee even gave Billy Kilmer, Joe Tehismann and tight end Jean Pugett the day off after a morning workout yesterday, and told them to report with the rest of the veterans by 9 a.m. tomorrow.

Pardee still has not decided who will open at quarterback in the scrimmage but said Scott Gardner, Tim Ellis and John Hurle will be given equal playing time. The signals will be called from the bench, and relayed by messenger backs or receivers.

Both teams also plan to unveil their versions of the three-lineman, four-linebacker defense, though scrimmage rules do not allow blitzes.