Three days before the Washington Redskins' playoff game against the Detroit Lions, the injury-riddled offense plugged some of its holes yesterday. Gimpy fullback John Riggins ran strongly in practice and Alvin Garrett, who will be starting for the first time at wide receiver, got a full day's work.

Garrett, the 5-foot-7, 178-pound former San Angelo State star who is filling in for Art Monk, although he has caught just six passes in three years as a professional, may not be the only new face on offense Saturday. Joe Bugel, the line coach, said it is "50-50" that Fred Dean will replace Mark May at right guard.

"I think Fred will start," said May, the 1981 No. 1 draft choice who was moved from tackle to guard this season. "They said they were going to rotate all three of us (May, Dean and left guard Russ Grimm), so all three of us will play. I would think I'd play 50 percent of the time. But Fred has been doing well; he deserves the opportunity."

Most of the uncertainty lurks in the backfield, where Riggins, Joe Washington and Clarence Harmon all have leg problems. As a precaution, Coach Joe Gibbs said he is preparing Wilbur Jackson, who has been on injured reserve since the season's first game, with the likelihood Jackson will be activated before Saturday.

Riggins, who did not play last week and only sparingly the previous week, took a full practice load yesterday, testing his pulled thigh muscle. But Washington, who has a sore right knee, had his work limited.

"They felt it was better for me to go light today," Washington said. "I woke up Saturday night in quite a bit of pain and I knew things weren't just right. But it feels better. I'll play Saturday. This is the playoffs. There is no way I'm going to miss this game."

Harmon is listed as doubtful with knee and hand injuries, but he ran well in the workout and should be available Saturday for his normal third-down passing duties.

"I feel a little better about how things are," said Gibbs. "We have our game plan set. We are trying to do a little extra every day to get ready, since we have a short week."

For Garrett, yesterday's practice was a unique experience. Normally, he is the Redskins' fourth receiver, which gives him little to do. Now, he is an integral part of the game plan, mainly because Virgil Seay, the No. 3 receiver, still is recovering from a pulled hip muscle.

"I think maybe all Alvin's needed is a chance to play and start in this league," Gibbs said. "He's a little guy with real good speed, excellent timing and he can jump. He's a total player, a real tough guy. He's had lots of seasoning, he's capable of going in and playing well. You can't consider him an inexperienced guy. He'll try to knock anyone around."

When Monk was hurt early in Sunday's game, Charlie Brown moved to his spot and Garrett took Brown's, catching his first pass for the Redskins, for six yards.

Garrett, one of the team's pleasant surprises in last summer's training camp, was acquired in the 10th week of the 1981 season when the New York Giants put him on waivers. He was San Diego's ninth-round draft pick in 1979, but was released in training camp. The Giants signed him in 1980 and he played 15 games before fracturing ribs. He gained 914 yards catching passes and returning punts that season; against the Redskins he caught fives passes for 69 yards--32 coming on a touchdown--and ran back a punt 66 yards.

When Gibbs was San Diego's offensive coordinator, he scouted Garrett, who was the most valuable player as a running back in the 1978 NAIA national championship. Gibbs and Wayne Sevier, the Redskins' special teams coach, remembered Garrett's special teams ability. That was the major reason he was signed last season. Then he broke an arm and was placed on injured reserve.

Garrett admits that lack of size has been his major stumbling block in the pros. But he says he is convinced "I can play with the big boys. I've never really gotten a chance."

May said he is taking this demotion much more philosophically than he did last season's, when Joe Jacoby replaced him at right tackle.

"I've matured," May said. "I also know I'm not playing badly. It's just that Freddie is playing really well and the policy with the offensive line is, if a guy is playing well, he should start."

May even went to Bugel last week and told him not to hesitate to use Dean. "I wanted him to know I wouldn't get depressed like last year," May said. "It's not that way anymore. I want what's best for the team."

Dean did not play last season because of an arm injury. In 1980, he started at both guard and tackle. He replaced Grimm at times during the New Orleans game two weeks ago and was a standout. Then he played extensively for the banged-up May against St. Louis, in part because Bugel said the Redskins wanted to take advantage of Dean's quickness on screen blocking.

"We'll go this week with the guy who fits best in our game plan," Bugel said. "Fred is a nifty runner, he's quicker than Mark. But Mark is magnificent against four-man fronts like Detroit plays. You put him in a phone booth, with a guy playing over him, and he battles and scratches. The way we handle it is, we give battlefield promotions. We go with the hot man."

Kick returner Mike Nelms said his thigh pull is "at least 90 percent healed" and that he will play this week . . . Cornerback Vernon Dean practiced with a cast on his fractured wrist.