Quarterback Mark Rypien's nightmarish start turned into a nightmare of a different sort yesterday while the Washington Redskins were stumbling to a 19-15 victory over the Dallas Cowboys in front of 53,804 at RFK Stadium.

Rypien suffered a hyper-extended left knee in the second quarter when Redskins guard Mark Schlereth rolled former Redskin Dean Hamel into Rypien as he released a pass. He went down, got back up, then needed considerable help getting off the field.

Rypien could be out for as long as four to five weeks, Coach Joe Gibbs said immediately after the game. Still, that diagnosis could change after he's examined today. The knee was so sore that he had trouble straightening it for a lockerroom X-ray, and he spent the second half on crutches, unable to put any weight on the leg.

Rypien's injury turned the Washington offense over to second-year man Stan Humphries. With Jeff Rutledge ineligible to come off injured reserve until after next week's game in Phoenix, the Redskins last night were shopping for a backup, with veteran Gary Hogeboom the most likely candidate.

Once again, it was defense that saved the Redskins (2-1). Darrell Green's 18-yard interception return of a Troy Aikman pass in the fourth quarter was their only touchdown -- and the second in three weeks for Washington defenders. And safety Todd Bowles's interception with 1:03 remaining in the game on first and 10 at the Washington 40 ended the last-gasp drive for the Cowboys (1-2).

The Redskins sacked Aikman eight times, intercepted him twice, pressured him numerous other times and forced him to fumble. They held the Cowboys to one yard their first five possessions of the second half. And they needed every play because the Redskins' offense finished with 214 yards, only 40 during a dismal second half.

What they did have was reliable Chip Lohmiller, who kicked four field goals, including a career-best 55-yarder in the third quarter, second longest in Redskins history.

"Thank goodness for the defense," Gibbs said. "They won it. Our defense found a way to make the plays and they took it upon themselves to win it. They did it and the special teams and offense didn't."

Humphries, 25, becomes the fourth Redskins quarterback since Joe Theismann's career abruptly ended in 1985 with a broken leg. Gibbs calls Humphries the best pure passer he has ever seen and the Redskins say he has both a strong arm and quick feet. He's also hot-tempered and a bit arrogant, not bad ingredients for a slumbering offense. The downside is that his NFL career had consisted of 10 passes before yesterday afternoon.

"I feel bad for Mark, but this is an opportunity for me," Humphries said. "That's the way I have to look at it. I don't know if I was getting impatient, but I've been here two years. I feel like it's getting to my prime years and I really want a shot."

Humphries completed only five of 13 passes for 58 yards yesterday, failing to lift an offense that spun its wheels a third straight week. But he was just one part of a unit that dropped six passes and gained only 40 yards in the second half.

The Redskins didn't score an offensive touchdown for the first time since Week 9 last season -- a 13-3 loss to Dallas at RFK. Oddly enough, their best drive of the first half came just before Rypien departed.

The game was tied, 3-3, when he hit Gary Clark for gains of 12 and 17 yards and Art Monk for 15. It was on the play to Monk that Hamel crashed into Rypien.

"It was simultaneous, throw and hit," Rypien said. Doctors say he may need an arthroscopic procedure just to examine how much, if any, damage has been done.

But Rypien remained hopeful, saying: "I think tomorrow might be a good indication of how I feel. It's as stiff as it could be right now. But we don't have to be back into uniform until Wednesday. This kind of thing comes with playing hard. It was just a freak thing. If I can get out and play with soreness, I'll do it as long as it doesn't hinder my movement. I'm willing to do that and worry about things at the end of the season."

Humphries trotted on to the field and was greeted by a loud ovation. Rypien had again struggled, completing only eight of 17 passes and had been booed until that last drive.

Gibbs immediately went for a big play. On first and 10 at the Cowboys 24, Humphries threw for Ricky Sanders in the end zone. Sanders had the ball for a moment but couldn't hang on.

Not that it mattered. Earnest Byner was called for holding, getting one of the 11 flags that cost the Redskins an embarrassing 102 yards.

Humphries did move the Redskins to the 6, where he missed Monk. Lohmiller kicked a 23-yard field goal for a 6-3 lead.

"Overall, I was mediocre," Humphries said. "I hit some third downs, but could have made some big plays and didn't do it. I hope the people are more patient with me than they've been with Rip the last few games. The way I feel right now is this is my chance and I've got to get in there and make things happen. But it's not going to happen every play."

At the moment, Gibbs may be wondering if it will ever happen. An offense that was supposed to be so spectacular has had an awful start.

"We're not clicking on all cylinders," guard Russ Grimm said. "I don't know what it is. We've got to sit down and look at the films. We had penalties, drops, missed blocks, you name it. We've got to get on the same page in a hurry. The defense pulled this one out for us. They played their tails off."

The Redskins did put one drive together in the first quarter. That was 67 yards in 14 plays ending with Lohmiller's 37-yard field goal. On third and three at the Washington 32, Rypien hit Sanders (seven catches, 78 yards) for 15. On second and 11 at the Washington 46, Sanders picked up 12 on a reverse. Rypien then went across the middle to Sanders for 23.

But from the 19, Byner was stopped for no gain, Hamel knocked down Rypien and Monk dropped a pass. Lohmiller came in and kicked a 37-yard field goal and a pattern had been established.

The Cowboys tied it at 3, on Ken Willis's 33-yard field goal with 7:42 left in the first quarter. They drove only 22 yards for the score thanks to Ralf Mojsiejenko's line-drive 35-yard punt that gave Rod Harris a chance for a 12-yard return to the Washington 37.

After Rypien got hurt, Humphries hit Byner for 21 yards and Sanders for 17 to get the Redskins to the 9. Humphries was sacked on first down, had a pass tipped by Jim Jeffcoat on second down and missed Monk on third. Lohmiller came on for the 23-yarder with 52 seconds left in the first half.

Rookie Brian Mitchell fumbled the second-half kickoff and safety Ray Horton recovered for the Cowboys at the Washington 8. The Redskins stopped Aikman and Willis tied the game with a 41-yard field goal.

But the Redskins got the ball back and drove 57 yards to give Lohmiller a chance at a 24-yard field goal. Byner made the biggest play of the drive, gaining 15 yards on a draw, then getting another 15 when linebacker Ken Norton Jr. grabbed his face mask. Humphries connected with Clark for 17 and Byner got nine to the Dallas 7.

But Kelvin Bryant dropped a third-down pass and Lohmiller gave the Redskins a 9-6 lead.

Then on third and 21 from the Dallas 39, Bowles knocked the ball from Aikman. Fred Stokes recovered at the 32 and three plays later, Lohmiller hit the 55-yarder.

On Dallas's first possession of the fourth quarter, Green stepped in front of Kelvin Martin, intercepted a pass, reversed his field and eventually got into the end zone.

"I must have run 200 yards," Green said. "It just happened so quick, I don't know what happened."

The Cowboys said one thing that happened was that Green held Martin and Coach Jimmy Johnson sprinted on to the field to tell officials about it.

"They mugged him," Johnson said.

That made it 19-6 but the Cowboys weren't finished. Aikman took them on a 71-yard, 11-play drive that ended with Emmitt Smith's two-yard touchdown run to make it 19-13 with 3:03 left in the game.

A penalty on the kickoff backed the Redskins deep in their territory. A sack pushed them to the 1. On fourth down Mojsiejenko took a safety by stepping out of the end zone rather than punt and give Dallas good field position. Shortly after the ensuing free kick Bowles intercepted Aikman to stop the Cowboys a final time.

"Dallas deserves a lot of credit," Gibbs said. "They've come a long way and have a good team. We know we have a lot to work on. We still are making some mistakes and it's hard to explain."

With the Redskins hokding a precarious 12-6 lead early in the fourth quarter, defensive end Charles Mann pressures Dallas quarterback Troy Aikman (top). Aikman gets off pass, but cornerback Darrell Green yanks it away from intended receiver Kelvin Martin (center). Then it's off to the races as Green weaves his way 18 yards to Redskins' only touchdown.