The Redskins' growing concern over Mat Mendenhall's slow-healing knee was eased today when tests on the injury proved negative.

Mendenhall was examined by Dr. Stanford Lavine in Washington. He now will continue rehabilitation of the knee, which was twisted last week. He had been expected back by the start of practice this week, but now he probably won't resume practice until at least Monday.

Mendenhall, a second-round 1980 draft choice out of Brigham Young, is a contender for a starting defensive-end position this season after spending last year on the injured-reserve list. The Redskin coaches say he is missing valuable teaching time this week, one reason they wanted to find out if the ailment was more serious than first believed.

John Riggins began answering questions about his determiniation today with some crunching blocks during the morning workout. Riggins appear in excellent condition despite his year's layoff. . . Defensive end Fred Cook, acquired from Baltimore last week, is expected to be at camp Wednesday after passing his physical today.

In a move that could enhance his chances of making the final roster, guard Gary Sayre, a fifth-round pick from Cameron State, was practicing long snaps today with Wayne Sevier, special-teams coach.

Sayre, a 6-foot-5, 265-pound strongman, has been making impressive strides at guard, but he could become a victim of roster space unless he can also become the center on punts and field goals.

Presently, Jeff Bostic handles those chores, although he, also, is being challenged by rookie Darryl Grant. At best, Bostic would be a third-team center, and the Redskins might prefer to carry another guard, like Sayre, and go with two centers.

"Gary is coming on fast," said Joe Bugel, offensive line coach. "He's so darn strong and big. He's earning more playing time each day. He's a great prospect, no doubt about it."