The Washington Redskins traded disgruntled cornerback Lemar Parrish to Buffalo yesterday after using their top draft choices to select players who they predict will give immediate help in two problem areas, cornerback and wide receiver.

Parrish, a former all-pro who asked to be traded because of a conflict with the defensive coordinator, Richie Petitbon, was sent to Buffalo in exchange for a fifth-round pick in this year's draft and a fifth-round selection in 1983, if Parrish plays half the Bills' defensive downs.

Earlier, Washington, which did not have a No. 1 selection, chose cornerback Vernon Dean of San Diego State (5 feet 11, 180 pounds) in the second round and wide receiver Carl Powell of Jackson State (6-0, 182) in the third. Coach Joe Gibbs said that Dean, an aggressive tackler, and Powell, a bona-fide deep threat, could wind up as starters this season. He also said those choices would probably allow him to keep Mike Nelms at safety instead of switching him to receiver, as had been planned.

Gibbs and General Manager Bobby Beathard were also optimistic about their other selections yesterday. In the fourth round, they took defensive end Todd Liebenstein of Nevada-Las Vegas (6-7, 250); in the fifth, they used the pick they got in the Parrish trade to select tight end Michael Williams of Alabama A&M (6-4, 220); and in the sixth, they took linebacker Lemont Jeffers of Tennessee (6-2, 210).

Parrish, 34, a 12-year veteran, had been upset that Petitbon kept him on the bench last season after the cornerback had returned from early knee surgery. Since coming to the Redskins in 1978 from Cincinnati (along with Coy Bacon in exchange for a No. 1 choice), Parrish had intercepted 21 passes to rank as one of the premier cornerbacks in the league. But the Redskins were concerned about his age (he was the oldest player on the team) and what influence he might have on squad morale if he were unhappy.

"You are better off going in a different direction if a guy doesn't want to be here," said Gibbs, who thought the Redskins received adequate compensation in the trade for a player of Parrish's age. "He is a talented player, but it was obvious he really just wasn't happy here. We didn't give him away. We got a talented prospect (Williams) in return. I think this is the best for us, for Lemar and for Buffalo."

Parrish, speaking from his home in Florida, said he was "delighted with the trade. I'm very excited about it. I am happy now. I just want to play football and there are no problems with the Redskins any more. I want to thank Bobby Beathard for working this out with me. He's always been fair and I think this is the best thing for the Redskins and for me. Now I'm going to prove that I can still play football."

Buffalo first made inquiries about Parrish on Monday, but the trade was not completed until midway through the fifth round. The Bills had been seeking a cornerback in order to shore up a questionable secondary. The Redskins had been trying for two years to deal Parrish.

This was not a good draft for cornerbacks, so the selection in the second round of Dean has to be viewed with some caution. But with the Parrish trade in the works since Monday, Washington knew it had to get a cornerback early.

The Redskins, who had Dean rated as the third best cornerback available, believe he will add aggressiveness to a secondary that Gibbs thought was much too timid last season, especially against the run. With Parrish gone, Dean will be challenging both Joe Lavender, who is better on pass coverage than tackling, and Jeris White, a better force man, for playing time.

Powell was a seldom-used receiver (29 career catches) at Jackson State, which employed a wishbone offense. But he runs 100 yards in 9.4 seconds and gives the team a much-needed deep threat with size to complement Art Monk. Powell's contribution next season will depend on how fast the team can develop his raw talents.

The Redskins entered the draft hoping to select a top-notch defensive lineman, although Beathard was almost certain all the best ones would be taken by the time he had a chance to pick. He was right, so Washington switched its focus to its other major needs: cornerback, wide receiver and tight end.

Beathard said yesterday he had hoped until late Monday night that cornerback Bobby Watkins of Southwest Texas might last until the 49th pick. But Detroit chose him three turns before the Redskins, who then had to select between Dean and linebacker Clayton Weishuln of San Angelo State.

"We went with our greatest need," Beathard said. "We think Weishuln will be a great linebacker but with Larry Kubin and Neal Olkewicz in the middle, we thought we were better set at linebacker than at cornerback."

Petitbon: "Dean is a good, solid, all-round player. He can come in and help us right away. Now, that doesn't mean he will start because the two guys starting now are good. But he is further ahead than most rookies."

Dean, who also is a talented kick returner, intercepted six passes last season to lead the Western Athletic Conference and rank fourth nationally. That was his only year of major college football after redshirting in 1980 when he broke an arm after two games that season. Before then, he spent two years at Los Angeles Valley Junior College and a season with U.S. International University.

"I almost quit football after junior college," Dean said yesterday. "I wanted to get married because that sounded fun and exiting. But I'm not with the same lady anymore, so you can see how exciting it was. I never did get married and USIU called and wanted me to play, so I did."

Powell probably isn't that much faster than little Virgil Seay, the other starting wide receiver last year. But he is at least four inches taller than Seay and has what Dan Henning, Redskin assistant head coach, calls "a great burst to the ball. He has all the tools we wanted: size, speed and he can jump. He's also easy to work with.

"He's going to need quite a bit of work, which is probably the reason he wasn't taken higher. But we knew that going in and we thought it was a plus that he seems to take to coaching very well. He's someone that can get off the ball and go deep, and that's what we need."

Liebenstein, described by Beathard as a decent pass rusher, had knee surgery four games into last season, the main reason he was left until the fourth round. Beathard said that if Liebenstein improves his upper body strength prior to training camp, he could challenge for playing time along a defensive front that delivered little pass rush last year.

"The main thing is, his knee is healthy," Beathard said. "We are sure of that. He can be a fine player."

Williams was the Redskins top-rated tight end in the draft. "I think he could come in and be a starter if he can beat out Rick Walker," Gibbs said. "He is a big rawboned kid who has gone from 228 to 235 and could be even bigger. He has real big hands and he runs nifty pass patterns, just what we are looking for in our offense."

Jeffers, the Redskins' last pick yesterday, needs to gain at least 15 pounds, Beathard said. "He is undersized, but he is quick and he covers well on passes. He can carry 225 to 230 pounds. It'll be just a matter of how hard he works in our weight program."