Washington Redskins Coach Joe Gibbs said yesterday that rookie Brian Mitchell will make his debut at running back Sunday afternoon against the Miami Dolphins at RFK Stadium. Gibbs emphasized he wasn't unhappy with Earnest Byner, who will continue to start and get most of the carries.

But searching for answers to a problem he hasn't faced often during a decade in charge of the Redskins, Gibbs said Byner might need an occasional "breather," and that he was as curious as anyone else to see what the speedy Mitchell will do.

A fifth-round draft choice and converted option quarterback, Mitchell has been one of this season's bright spots. He was drafted because he had speed and terrific hands, and because the Redskins thought he could be a third-down back in the mold of the Giants' Dave Meggett.

Yet in excelling at both kickoff and punt returns, he has looked so good that they believe he might now be more than that. On a team sorely lacking speed, he may add an exciting dimension. His only time in the backfield was at quarterback against the Eagles when Stan Humphries and Jeff Rutledge were injured.

Told he would get the ball on Sunday, Mitchell smiled and said, "I hope I do something with it . . . I'm very anxious to get back there. I feel I'm ready, but I've tried not to worry about when a chance would come. I didn't want to get too anxious and let it affect the things I'm doing now."

Byner and Gerald Riggs had split time almost evenly until Riggs suffered a sprained arch three games ago. Since then, Byner has gotten every carry, beginning with a spectacular performance against New Orleans -- 116 yards on 26 carries.

He gained only 39 yards on 14 carries against Dallas. In that game, the Redskins fell behind, 10-0, and Gibbs abandoned the run and called 54 passes.

Gibbs isn't sure splitting time between two backs will make a difference, but in the aftermath of a bitter loss, he is willing to try.

"Earnest went all the way against the Saints and played great," Gibbs said. "I don't know that that's the answer. What we've got to do is get Brian in there and see what he can do."

This week, as the Redskins have re-evaluated so many things, Gibbs has publicly scolded himself for giving up the running game so early against the Cowboys. He has reminded himself that running games are built on repetition, and that big teams (like Washington) tend to wear down small teams (like Dallas), especially in the second half.

At the same time, he has three of the game's best wide receivers and "I have a tendency to want to hit a big play. We're so good in the passing game that sometimes when we fall behind, I go to it too quickly."

Gibbs likely will always be torn by that argument, and it's extraordinary to hear an NFL coach publicly second-guess himself. Part of that second-guessing was a promise yesterday that, almost no matter how the Redskins start Sunday against one of the AFC's best rushing defenses, the running game won't end after a dozen carries.

What must worry him is that Thanksgiving Day in Dallas wasn't the first time the Redskins have come out and been unable to fire opponents off the line of scrimmage. They knew this was a season when they were in transition in several areas, but with a big, strong offensive line and two quality backs, they always expected to run the ball, especially with teams so aware of wide receivers Art Monk, Gary Clark, Ricky Sanders and the passing game.

Yet three times in the past five weeks -- against the Giants, Eagles and Cowboys -- the Redskins have gotten almost nothing on the ground. Does that mean their line -- a mix of old and new Hogs -- isn't as good as they thought? Does that mean that Byner and Riggs, who are middle-aged running backs by NFL standards, are on the downside of their careers?

"It's a combination of things with the running game," Gibbs said. "It has been like our offensive team -- very inconsistent. We rushed the ball against the Saints better than I thought we could. They're hard to run on and very good. Then we turn around against Dallas and don't rush the ball at all. We didn't get it done against Philadelphia. We did one game against the Giants, but not the other."

It's especially confusing because at times the Redskins have run the ball better than at anytime in the post-Riggo era. They are averaging 3.9 yards per carry, and since John Riggins retired in 1985, they've done better than that only once. They may sputter occasionally, but their 1,202 yards put them on a pace for 1,748, which would be better than they did in 1986 or 1988.

But the numbers have varied wildly -- six games of 100 or more yards and 29 or more carries and five games of 21 or fewer carries and 87 or fewer yards. They are 5-1 in the big yardage games, 1-4 in the small ones.

"It could be any one of several things," Gibbs said. "Effort is one factor. It's also game plan, missing a block here and there or a running back making a great play. The Dallas running back {rookie Emmitt Smith} made about six great runs against us and they built up some numbers {162 yards}. We didn't. You could say a thousand things. The only thing I'm thinking is that it's over with. We can learn from it and practice hard this week and get ready for this game."

One scout said the Cowboys had clogged up the middle and that the Redskins simply weren't fast enough to get outside. "One of the best things Earnest does is bounce things outside," Gibbs said. "He's good at that and he can get the ball on the edge for us."

The Cowboys did such a good job stopping the run that they were able to drop seven and eight men into pass coverage. That, in turn, led to Mark Rypien attempting 54 passes and often throwing into double- and triple-coverage.

"I think what happens is you have a guy here and a guy there breaking down," tight end Don Warren said. "It's not the same guy, but when you have inconsistency, the running game doesn't go all the time. I've been at fault, other guys have been at fault, it's been across the line. You fix it by hard work in practice and a lot of hustle and trying to get the right footwork down. Blocking is all footwork. If you get it, you get the running game going."

The Redskins addressed the problem methodically, first going into meeting rooms and watching films, then going onto the field and having a pair of long, tough practices.

"You have to stay at it," tackle Jim Lachey said. "We got behind and tried to catch up with the pass. We need more attempts {to} get the chance to wear them down. It's hard when you're only running 15-20 times a game. The running game will sometimes click in the third or fourth quarter when it hasn't gotten it done earlier in the game."