The Washington Redskins placed left defensive end Mat Mendenhall on a nonfootball illness reserve list yesterday, six days after he left the team without notifying officials and returned to his Utah home.

On a day when National Football League rosters had to be trimmed to 60 players, the Redskins (who started the day with 62 players on the active roster) reached the limit by also placing rookie offensive tackle Todd Hallstrom on the injured reserve list because of a staph infection above his right knee.

By placing Mendenhall in this sort of paperwork limbo, the Redskins answered a roster question, if not the questions as to why Mendenhall left the team.

"We put Mat in a reserve category until he gets his personal problems settled," Coach Joe Gibbs said. Gibbs said he has talked to Mendenhall by telephone twice since Mendenhall left. Pressed on the subject, Gibbs said pointedly, "Mat has got some personal problems. What else do you want us to say?"

The Redskins will retain the rights to Mendenhall for an indefinite period. When or if the third-year player from Brigham Young returns to the team, the Redskins would have to subtract another player.

Getting Mendenhall's side of the story has been difficult. He has either refused comment or not returned phone calls to his home in American Fork, Utah. Neither Mendenhall nor Marv Demoff, his Southern California-based agent, returned calls yesterday.

When Mendenhall returned to Utah on Thursday, his wife Julie said that he had left the team because of contract problems. Mendenhall is in his option year. He was scheduled to earn about $110,000 for this final year of his three-year contract.

General Manager Bobby Beathard said yesterday that Mendenhall's contract had nothing to do with his departure. Beathard said he and Demoff had reached a verbal agreement for a new contract prior to training camp. He said that Mendenhall had not signed the contract because he had not talked about it with Demoff.

Last year, Mendenhall started every game at left defensive end. He made 34 tackles in the regular season, seven more in the postseason.

On Aug. 15, however, Mendenhall was told by Lavern (Torgy) Torgeson, defensive line coach, that Todd Liebenstein would start ahead of him in the preseason game against Miami.

"I explained to him that Todd and (right tackle) Darryl Grant were going to start, but that it didn't mean anything, that it was only a preseason game," Torgeson said yesterday. "Mat seemed to understand."

Two days later, Mendenhall left. When training camp roommate Clint Didier returned to the dormitory room Wednesday evening, Mendenhall had already left. His closets and drawers were empty.

Those Redskins players who were close with Mendenhall say he was bothered by the fact Liebenstein, a second-year player, was moved ahead of him. They say he was bothered by reading the newspapers, seeing Redskins coaches' comments about his battle with Liebenstein. They say he talked about leaving the team several days before he left. One time, they thought they had talked him out of leaving.

"I remember Mat and I were walking from Dickinson College (Redskins' training camp in Carlisle, Pa.) over to the lunchroom and Mat said 'They are trying to give my job to Todd,' " said Dexter Manley, right defensive end.

" . . . I could tell that moving Todd (to starting left end) had more affect on Mat than any contract problem or personal problem . . . I think Mat was a little upset. When Todd is out there taking his reps (practice repetitions), well, anytime you were a starter like Mat, quite naturally it upsets you . . . But we all have to face reality. Only one man can get the job. No, I wasn't surprised at all when he left. I had an idea he might."

"I had a feeling something was bothering him," Torgeson said. "It was something I couldn't put my finger on. This was Mat's third year with us and after three years, you get a feeling that you know someone. When a guy starts and then you tell him that he is not going to start, usually he bounces back. We wanted to see what Todd could do."

Torgeson said the Redskins will likely use Liebenstein at left defensive end on rushing downs--like Mendenhall, Liebenstein is most efficient against the run--and 13-year veteran Tony (Mac the Sack) McGee on passing downs. Rookie Charles Mann also figures in the plans, Torgeson said.

Quarterback Joe Theismann said of his absence from Sunday's practice: "It was because of a personal business. Evidently, it was just a bit of a misunderstanding. It was something that I told them about a long time ago . . . (Missing practice) is not something I would do during the regular season. You just have to understand what time of year it is."

Gibbs met with strong safety Tony Peters in his Redskin Park office Monday, talking about personal matters. Peters is facing nine counts of cocaine distribution and possession and is currently on a paid leave of absence from the team.

Wide receiver Art Monk, sidelined with a sprained medial collateral ligament in his left knee, is not progressing as fast as the Redskins had hoped. "I talked to Art and he said he feels like it hasn't improved much," Gibbs said. "Right now, it doesn't look encouraging."

Free safety Greg Williams, seemingly recovered from a torn knee ligament suffered early in training camp, is working out with the team again . . .

Officials from both the NFL Players Association and the NFL Management Council said yesterday that the two sides' separate fact-finding missions into kicker Mark Moseley's formal grievance against the Redskins have not yet started. Moseley filed a grievance to collect $30,000 in incentive bonuses that he feels he earned, but hasn't received.