Rookie Brian Mitchell's debut at running back was overshadowed by Earnest Byner's 157 yards, but it was another bright spot in the Washington Redskins' 42-20 victory over the Miami Dolphins yesterday at RFK Stadium.

Mitchell entered the game for two plays midway through the second quarter and picked up gains of nine and four yards. He finished with 36 yards on seven carries.

While showing flashes of the outside speed the Redskins need, he fumbled once, caught a pass (for zero yards) and had a 16-yard run in the fourth quarter.

Most of his day was spent returning punts and kickoffs, and he said he realizes that at least for a while his only carries in the backfield will be as a spot substitute.

"It felt real good," Mitchell said. "It was a chance to do what I wanted to do. There were some big holes out there and I was concentrating on not trying to do too much. It gave me some confidence just getting out there, and I hope if Earnest is tired, they'll keep giving it to me." Caught Off Guard

When Art Monk caught a six-yard touchdown pass from Mark Rypien early in the second quarter, it broke a string of 21 quarters the Dolphins had gone without allowing such an occurrence. Miami entered the game having yielded only 11 points per game. . . .

On the previous worst defensive games, the Dolphins allowed the New York Jets 342 yards overall and the Los Angeles Raiders 177 yards rushing. The Redskins had 467 total yards, including 222 on the ground. The Patriots scored 24 points in a 27-24 loss to the Dolphins in Week 1, and until yesterday that had been the most points scored against Miami.

Unmarked Man

Even though the Redskins scored the first 21 points of the game, Coach Joe Gibbs said: "We were never really comfortable the whole time. You always feel with {Dolphins quarter Dan} Marino, it's just a matter of time {before he strikes}. We rushed a three-man line against him at times because we didn't think we were going to get to him anyway."

Marino was sacked once.

Injury Report

The Redskins had only two injuries: Center Jeff Bostic sprained his right thumb and guard Russ Grimm sprained his right shoulder. Neither injury is believed to be serious, but both sat out much of the second half as the Redskins used an offensive line of Jim Lachey and Joe Jacoby on the left side, Raleigh McKenzie at center and Mark Adickes and Ed Simmons on the right side. Jacoby hadn't played guard since his rookie season (1981). . . .

The Redskins say the time printed on tickets for Sunday's game with the Chicago Bears is incorrect. The game will start at 4.