Defensive end Mat Mendenhall, a three-year veteran and a starter on the Washington Redskins' 1982 Super Bowl championship team, was formally waived yesterday from the Redskins' injured reserve list.

Mendenhall, who left training camp for personal reasons last season, entered camp this year requiring surgery to remove bone chips from his fractured elbow and was placed on injured reserve July 31.

The Redskins also waived wide receiver Rodney Goosby from the injured reserve list. Goosby, signed as a free agent in 1982, hurt his hamstring in training camp.

Defensive tackle Bob Slater, who was injured in preseason and recently returned to practice, underwent surgery to remove cartilage from his left knee yesterday and will not play this season.

The news was better for most of the other injured Redskins. Strong safety Ken Coffey, who went to the hospital Monday for gastrointestinal tests for stomach pains, was released after the tests proved negative. He is expected to resume practice today.

Running back John Riggins, who sat out Sunday's victory over Detroit with a back injury, is likely to be listed as "probable" for the game this Sunday at Philadelphia, according to Charlie Taylor, the Redskins' public relations director. Riggins is expected back at practice this week.

Wide receiver Calvin Muhammad (bruised ribs) and left guard Russ Grimm (slightly separated shoulder) also are expected to be probable for the game, although Grimm may be held out of some plays in practice.