The Washington Redskins got a bit of good news yesterday from defensive end Fred Stokes, who said the soreness in his sore left shoulder was substantially less than it had been on Monday.

Stokes was so improved that he'll dress for this afternoon's practice and see how much contact he can tolerate. Trainer Bubba Tyer has an array of new braces and harnesses and hopes one of them will allow Stokes some freedom of movement while holding the shoulder in place.

Stokes is still listed as questionable for Sunday's game with the San Francisco 49ers, but the Redskins are hopeful he'll be able to play.

His ability to play long stretches became less important yesterday when the Redskins acquired veteran Eric Williams from the Detroit Lions.

Williams is expected to be at Redskin Park when the Redskins return to work on Wednesday and probably will play some on Sunday.

General Manager Charley Casserly telephoned Alonzo Mitz on Monday to say the Redskins might need him. He got another alternative late Monday when the New Orleans Saints waived Pat Swoopes, but he favored Williams to either of those alternatives.Quietly Doing the Job

The offense gets more attention, but the Redskins' defense has been the more impressive unit, having allowed just one second-half touchdown in its last eight regular season games. It's given up 10 second-half points in that stretch, and in six of the eight games didn't allow any. The San Diego Chargers scored the only touchdown, the Atlanta Falcons the only field goal. The Redskins won both games.

The Washington defensive starters averaged 24.8 years per man in the opener, compared with 29.1 years per man in last year's opening game. This year's lineup included rookie linebacker Andre Collins, who is 22, and three other starters 25 or under: Martin Mayhew (24), middle linebacker Greg Manusky (24) and defensive tackle Tracy Rocker (24).

Factually Speaking

Mark Rypien has 11 touchdown passes and four interceptions during the Redskins' six-game winning streak. . . . The 49ers hold an 8-6-1 series edge over the Redskins and have won three of the last four regular season games, including the last two at Candlestick Park, where the Redskins haven't won since 1976. . . . The Redskins and 49ers were the only NFL teams to finish in the top 10 in offensive and defense last season. . . . The Redskins are 7-0 when Earnest Byner is starting running back.