CARLISLE, PA., AUG. 15 -- The Washington Redskins received more bad news from an already thin defensive line today when the left shoulder of defensive end Fred Stokes once more popped out of place.

Stokes was injured during a morning one-on-one drill with tackle Jim Lachey and left the field, head bowed, left arm cradled against his chest. He was sent back to Washington to be examined by team physicians and a more complete diagnosis will be available Thursday.

Stokes had returned to practice Monday after missing 11 days when the shoulder popped out during a scrimmage with the Pittsburgh Steelers. He took treatment and did strengthening exercises, and while saying he felt better, team officials warned there was a chance of a recurrence.

Now they hope to find the cause of the problem, which could range from bad luck to some sort of damage that will require surgery and a long-term rehabilitation. An MRI test was performed tonight and doctors were to meet with Stokes Thursday morning.

At the very least, his availability for the Sept. 9 season opener against Phoenix is very much in doubt.

"This is bad news," Redskins defensive coach Richie Petitbon said. "This is an area where we were already thin. Right now it's not a good situation."

Stokes's injury is a serious blow to a defense that already has problems. The Redskins again will be without three-fourths of their starting defensive front for Friday's preseason game against the Steelers at RFK Stadium and are very thin in the secondary.

Left end Charles Mann will be in his usual spot but Stokes, left tackle Tracy Rocker (strained biceps tendon) and right tackle Darryl Grant (contract holdout) won't play. Markus Koch will make a second straight start at right end.

The middle will be muddled. Rookie Kent Wells probably will get another start at Grant's right tackle spot and Plan B signee Milford Hodge will start at left tackle. Veteran free agent Alonzo Mitz also will play.

That defensive line played poorly in Saturday's 31-27 loss to the Atlanta Falcons, who gained 406 yards, including 173 in the fourth quarter.

General Manager Charley Casserly has spent much of his spare time phoning other teams to determine what might be available. He has not come close to making a trade and didn't seem to think he would.

"We're not going to make a trade," Coach Joe Gibbs said flatly. "There's just no one out there."

Stokes took the news hard, saying, "It's the same thing as before. It didn't stay out as long and wasn't as painful, but it's the same type feeling."

He said often while sidelined that he had to be careful about doing too much too soon and today was scolding himself for doing just that.

"I just tried to overdo it," he said. "I don't think I came back too soon. I felt great. But maybe I should have backed off a little on some things. I knew this was a possibility. Next time I'm going to make sure I'm ready. I'm going to take my time and be careful. It's frustrating, but this is preseason and I still have time to get well."

Redskins trainer Bubba Tyer said it was unlikely Stokes returned too soon or did too much. Instead, Tyer said Stokes has an injury that is hard to diagnose and harder to prevent once it occurs.

"When you stretch that joint, there's a high percentage chance of it occurring again," Tyer said. "It's something you just have to work on and get stronger. We're sending him back to get a more precise idea about what is wrong."

Stokes, 26, was one of the most coveted of the first class of Plan B free agents two years ago and he chose the Redskins over several other teams, in part because of a three-year, $830,000 offer.

He played very well last season and was inserted into the starting lineup for the final five games, after Dexter Manley was suspended and Koch was hurt. Stokes was credited with 24 tackles, three sacks and seven quarterback hurries.

The Redskins had counted on him at right end this year and thought that with him and second-year man Rocker, their defensive line would only get better.