Darrell Green's 18-yard interception return for a touchdown early in the fourth quarter yesterday broke open Washington's 19-15 victory over Dallas.

It was the second time Green has returned an interception for a touchdown; the other time came in 1984 against the Cowboys. This time Charles Mann got pressure on quarterback Troy Aikman and Green stepped in front of wide receiver Kelvin Martin. Safety Alvin Walton leveled Emmitt Smith on the return.

"Me and the wide receiver were banging heads," Green said. "The quarterback was chased toward me. They'd done the same thing in the first quarter and we got a knockdown."

He cradled the ball under his arm, one more trophy for a defense that has played better on the field than it looked on paper. "This is my eighth year," Green said, "and we've never had a glamor defense. We have a bunch of guys hustling and playing hard." Injury Report

Mark Rypien's knee injury is the most serious, but several Redskins were hurting at the end of the game.

Defensive lineman Eric Williams, acquired from Detroit two weeks ago, made his debut in a few passing situations. But he sprained his left shoulder during practice last week and said it bothered him several times yesterday.

"The doctors are going to look at it again," he said. "I might have to rest it. I probably should have today, but I just had to get out there."

The injury list also included running back Earnest Byner (knee sprain), defensive lineman Fred Stokes (dislocated middle finger), linebacker Kurt Gouveia (sprained wrist), linebacker Andre Collins (quadriceps contusion) and long snapper John Brandes (sprained neck).

Cowboys defensive back Stan Smagala broke his left arm and offensive tackle Mark Tuinei sprained an ankle. . . .

Rypien was injured when defensive tackle Dean Hamel slammed into his leg. Hamel used to play for Redskins and was one of Rypien's golf partners.

Redskins guard Mark Schlereth "had ahold of my jersey when I saw" Rypien, Hamel said. "I just threw myself into him. I didn't know I landed on his leg. When I got up, I turned around and he was limping off the field. I said, 'Whoa.' I didn't know what had happened."

Said Rypien: "I have no idea what happened. I mean I didn't go immediately to the replay room and ask for it. . . . I saw Dean at the end of the game, and he said he was sorry. I didn't know what he was talking about." Doug Williams Not in Plans

Asked whom he was likely to sign to back up Stan Humphries while Rypien heals, General Manager Charley Casserly said: "We want someone who was in an NFL training camp this year."

Did that mean he had no interest in Doug Williams? "We want someone who was in an NFL training camp this year," he reiterated.