Hours earlier, Washington Redskins quarterback Mark Rypien was being booed by fans at RFK Stadium. Now, as he was leaving the locker room with the aid of crutches and wearing an immobilizer on his sprained left knee, Rypien was being cheered.

"Head up, Rip!"

"See you back out there soon, Rip!"

The scene was repeated after Rypien hobbled up a flight of stairs and began making his way through the parking lot. As if healed by the enthusiastic greeting, he stopped leaning on the crutches and starting walking with a ginger, stiff-legged gait.

"It really helps," Rypien's wife, Annette, said. "It really helps him to know people are still backing him, that they're still behind him."

But the cheers will not help Rypien return to the starting lineup. That will be a matter of time and treatment. How much time and how much treatment was not clear shortly after Rypien's replacement, Stan Humphries, and a tenacious defense had led the Redskins to a 19-15 victory over the Dallas Cowboys.

Rypien spoke hopefully about whirlpools and trying to start next Sunday's game in Phoenix against the Cardinals. But Redskins Coach Joe Gibbs talked about the possibility of Rypien being out four to five weeks, and team physician Charles Jackson talked about the possibility of arthroscopic surgery.

"Depending on how badly stretched the ligaments are, we might have to do an arthroscopy," Jackson said. "We can anticipate some trouble and soreness, and we might have to make further studies. . . . The question we have to answer is how much injury is there to the cartilage."

Rypien said all he knew was his knee hurt.

"It's just so stiff and sore right now," he said, "we can't really judge anything."

He was injured with two minutes left in the first half. Having released a pass to wide receiver Art Monk a split-second earlier, his left leg was planted when guard Mark Schlereth blocked Cowboys defensive tackle Dean Hamel, sending the former Redskin slamming into it just below the kneecap.

The completion to Monk made Rypien three of three for 44 yards on the drive, increasing his statistics for the game to eight of 17 for 97 yards. "It was about time we had gotten something going," said Rypien, who had several passes tipped at the line of scrimmage and had watched as teammates dropped several balls.

"Our defense was playing lights-out at that point, and we had to get something on the board before halftime. It was our turn to pitch in." But as soon as Hamel hit him, Rypien knew his turn was over.

"Your knee goes into a position it's never been in before," Rypien said. "It buckles the reverse way. You just say to yourself, 'I hope everything is tight and together.' "

That seems to be the case, according to Jackson, who pronounced the joint "very stable with no abnormal motion."

But the pain . . .

"When we examined him on the field, he was in a lot of pain," Jackson said. "At first, he was having so much pain, when we brought him inside we could not straighten the knee for us to take an X-ray."

The stability of Rypien's knee probably means the ligaments on the sides of the joint are okay. And Jackson said there was "very little swelling." Still, there are ligaments on the inside of the joint. That means there is a lot of uncertainty.

"Whether there is any posterior damage, in the back of the knee, they still have to check on that," Rypien said.

"Monday we'll see what's the extent of the irritation," Jackson said. "And we'll analyze the fluid."

Rypien has been through this before. He said that when he was a freshman at Washington State, he tore the medial collateral ligament in his left knee while playing pickup basketball. He had surgery and a three-month rehabilitation. And last season, during the first half of the game Dec. 3 against the Cardinals out there, he hyperextended his left knee.

That time, "at halftime, I was able to walk it off," he said. "They taped it and it felt a lot better, so I was able to come back in the second half. With this one, the fact that I couldn't move it kept me from being able to come back.

"I'd like to think that maybe I'll wake up tomorrow and it'll be real stiff, and I'll get in and do some whirlpool work and maybe by the afternoon I'll be getting a little movement back.

"Then maybe by Tuesday I'll be getting a little bit more, and then maybe by Wednesday I'll be able to go out and do some seven-on-seven or something to get myself going. That's the way I'd like to see it."

That's also a lot of maybes.

This is how Jackson sees it: "Every once in a while we're fooled in the subsequent days -- to the good and the bad."

Redskins quarterback Mark Rypien suffered a sprain, or hyperextension, to his left knee in the second quarter of yesterday's game with the Cowboys when he was hit right below the kneecap. Charles Jackson, the Redskins' orthopedist, said Rypien's knee was stable and there was no abnormal motion. Fluid will be taken from the knee today and further examination will take place to determine how much damage was done to the meniscus cartilage and ligaments.