After watching his team stumble in a 38-7 preseason loss to Miami at RFK Stadium last Friday night, Washington Redskins owner Jack Kent Cooke admitted yesterday, "They seemed flatter than a shadow.

"Oh yes, I am concerned, but I am always concerned, even when the team is winning," Cooke said. "You see, I have a low threshold of concern, so I worry about almost everything, particularly the Redskins.

"Having won the Super Bowl last year, I think most everyone fully expected we would breeze through the preseason and win all four games. Until we met Miami, I was satisfied with the team's performance. Miami worried me, as it worried everyone in the Redskins organization.

"But I think it is a positive worry," Cooke continued, "alerting not only the front office, but the coaches and the players to the need to be qui vive, the need to be awake. I think our performance will have a therapeutic effect."

The Redskins have a 1-2 preseason record and will play their last exhibition game against Buffalo Saturday at 6 p.m. For those who place significance in preseason results, Redskins officials are quick to note that the team was 0-4 in the preseason last year.

But the regular-season opener against the Dallas Cowboys is now just 13 days away.

The Redskins have acute concerns in the defensive secondary because of the absence of veteran cornerback Jeris White, still holding out for a raise in pay. When White had a contract dispute two years ago, he signed and reported on this very day, Aug. 23. Also, Pro Bowl strong safety Tony Peters is taking a paid leave of absence to prepare his legal defense against drug trafficking charges.

"I thought the team looked just fine against Atlanta and could have won it, but for a couple of unusually missed field goals by Mark Moseley," Cooke said about the Redskins' 13-10 overtime loss two weeks ago. "I thought they looked fine against Cincinnati, too (a 27-23 victory).

"I was not concerned after the first quarter against Miami. I thought we looked quite competent. Then, of course, the roof fell in on us. What happened, I don't know. The team, in toto, decided to take a vacation."

Left defensive end Mat Mendenhall, who departed Wednesday without notifying team officials, said yesterday from his home in Utah, "I have nothing to say and there is no telling when I will." Mendenhall had started every game last season but his job seemed in jeopardy this year.

Coach Joe Gibbs spoke with him this weekend and said the third-year pro left because of personal problems.

The Redskins must trim their roster by two players by 4 p.m. today to reach the 60-player limit . . . Offensive guard Mark May, who reinjured his partially torn right triceps, said yesterday, "I think I need to rest it for awhile. It's really a day-to-day thing."