CARLISLE, PA., AUG. 20 -- The Redskins had a scary moment this afternoon when linebacker Wilber Marshall was helped off the field after taking a helmet or foot on his left knee during the final practice play of the day.

Marshall lay on the field in pain for several moments and needed assistance to get into the locker room.

The preliminary report from trainer Bubba Tyer was that Marshall evidently suffered only a bruise and may miss very little time. Still, he'll be checked again to see about swelling and soreness. Less Time, Less Play

The National Football League's attempt to shorten the time of games has worked so well that some officials are worried about having done their job too well.

Last year the average time of a regular season game was 3 hours 11 minutes (it was 3:07 for a preseason game).

That's down dramatically this year. The average game was played in only 2:52 during the first week of the preseason. The Redskins have quickened their pace considerably, although at 3:05 they're well above the league average.

The NFL's goal was to keep the games right around three hours. Now, the worry among some officials is that the rule changes have shortened games too much, and therefore, eliminated too many plays (and points).

Last season the Redskins ran 69.75 plays per game. They've run an average of 55 in their first two preseason games, although almost everyone cautions against making a judgment too soon.

"We're keeping an eye on it," an NFL spokesman said. "It's too early to see a trend, but we're watching it."

The NFL had been worried about so many games running around 3 1/2 hours and among the new rules is that halftimes have been cut from 15 minutes to 12 and that the clock is started when the ball is spotted after incomplete passes and when players step out of bounds. Previously, the clock started when the ball was snapped, still the case in the final two minutes of the first half and the final five minutes of the game. Rocker Returns

The Redskins welcomed back defensive tackle Tracy Rocker at this morning's practice and say they expect him to play in Saturday's preseason game in Cleveland.

Rocker missed the first two games because of a strained biceps.

"I didn't like being out," he said. "I didn't come in here feeling comfortable that the job was mine. You never get too comfortable in this business, and nothing good can happen when you're not out there."