Brian Mitchell trotted onto the field, surveyed the scene and caught the eye of Philadelphia Eagles cornerback Eric Allen, who was laughing. "You're a quarterback?" Allen asked.

Mitchell nodded, smiled and proceeded to make his debut as an NFL quarterback. He got in for the last series -- eight plays -- of the Washington Redskins' 28-14 loss to the Eagles Monday night at the Vet, and when it had ended, he'd completed three of six passes, been sacked once and led the Redskins on a 56-yard drive for their second touchdown.

When he was done, several Eagles came over, patted him on the rump and said: "Good job."

That may have been an important moment in the career of Mitchell, a fifth-round draft choice from Southwestern Louisiana. He's a converted option quarterback and a guy the Redskins think might wind up in their backfield a lot in the next few years.

Mitchell may have enjoyed those few moments at quarterback, but what happened earlier in the game probably turned out to be more significant. Gerald Riggs was placed on injured reserve yesterday after suffering the same injury (a sprained arch) against the Eagles that sidelined him most of seven games last season.

Earnest Byner will continue as the starter, but Redskins Coach Joe Gibbs yesterday said Mitchell now will also get some carries.Stanley: 'Felt a Big Jerk'

Kick returner-wide receiver Walter Stanley appeared to have injured his right knee when he was tackled on a punt return in the first quarter. Stanley, who was placed on injured reserve yesterday, said that was only partly right. He said the blow that hurt him happened not during the tackle, but in the pile of bodies after the tackle.

"I think it was purposely done," Stanley told WUSA-TV. "I had already been tackled and felt the guy pulling on my leg as I tried to get up. . . . The play was blown dead and I felt a big jerk on my leg. That's what did it."Still Another Injury

Punter Ralf Mojsiejenko didn't leave the field on a stretcher, so he was in better shape than some of his teammates. Mojsiejenko suffered a sprained ankle midway through the second quarter when he fell on Roger Vick. He limped noticeably through the remainder of the game and was sent for X-rays yesterday. . . .

All the Redskins' injuries didn't occur during the game. Guard Mark Schlereth was on the two-man inactive list after suffering a severely sprained right ankle during Friday's practice.Ticket Correction

The Redskins say the game times on tickets printed for the Dec. 9 game with the Chicago Bears at RFK Stadium is wrong. The game will start at 4 p.m.