Washington Redskins wide receiver Calvin Muhammad, who sprained an ankle on the Redskins' first kickoff Sunday in Pittsburgh, was put on injured reserve yesterday for a minimum of four weeks and won't be back this season unless the Redskins (7-5) make the playoffs and decide to activate him.

To replace him, they re-signed rookie Joe Phillips, their leading receiver in preseason.

"It's too bad things haven't gone right," Coach Joe Gibbs said in announcing the decision on Muhammad. "Hopefully, he can get it turned around here."

Muhammad refused comment, saying he would not talk about his situation until today. He had been demoted from the first team. Then he put in brief appearances covering kickoffs.

Muhammad was not the only Redskin missing from yesterday's practice at damp, dreary Redskin Park.

Running back John Riggins was at home in Vienna in bed with the flu. After being examined by team doctor Donald Knowlan yesterday morning, Riggins spent a second consecutive day at home.

"He is off his feet, out of the rain and in bed," said his wife, Mary Lou.

It is uncertain if Riggins will return to practice today, but his illness is not expected to affect his availability for Sunday's 4 p.m. game with San Francisco (7-5) at RFK Stadium.

"You just have to see, with the flu," Mary Lou Riggins said.

Guard Russ Grimm and defensive tackle Tom Beasley did not practice because of sprained ankles but are expected to be ready Sunday.

On the offensive line, Grimm was missing, but tackle Joe Jacoby was back. Jacoby, who has missed the last six games with a sprained right knee, practiced at left tackle throughout offensive drills and appeared to perform well. "That was a big plus for us," Gibbs said.

Guard/center Jeff Bostic took Grimm's place at left guard, and Rick Donnalley, playing with a huge cast on his broken left hand, worked at center.

It has been no secret for weeks now that the Redskins have not been pleased with Muhammad, who has caught only nine passes for 116 yards and one touchdown, that in the second game of the season against Houston.

Since September, the numbers are startling: He has had only two receptions for 21 yards and has been shut out in six games.

Most puzzling is a comparison between last season and this. Acquired from the Los Angeles Raiders in October, Muhammad became the Redskins' second-leading receiver with 42 catches for 729 yards.

"I don't know what's wrong -- you tell me," Muhammad said in an interview earlier this month. "We both know I can't run the whole team."

Phillips, who caught 14 passes for 131 yards in the four preseason games, was visiting New York Giants running back George Adams, a friend and teammate from the University of Kentucky, when his agent called early yesterday morning and said the Redskins wanted him back.

Phillips, who has been looking for work in Lexington, Ky., was signed last week as a possible replacement for Malcolm Barnwell (who was waived), then released two days later when the Redskins decided on veteran Mark McGrath.

Phillips, who had been visiting Adams in New Jersey last week, went back again to Adams' house, then got the call to return.

"George told me, 'If they release you again, just come back,' " Phillips said.

Phillips, who is expected to play on special teams, said he is "rusty" now.

"It's been 12 weeks since I've played, and the Redskins have put in some new stuff," Phillips said. "It kind of makes you mad. You know you're not where you were . . . It's just gonna take awhile."

In Atlanta, three-time Pro Bowl guard R.C. Thielemann underwent successful reconstructive surgery on the interior cruciate ligament in his right knee, head trainer Bubba Tyer said.

Thielemann was injured in the Chicago game Week 4 and is out the rest of the season.