Beset by injuries and absences since the MLS season began, D.C. United suffered another setback yesterday when Brazilian forward Thiago Martins re-injured his left knee just days before he was to be available for the first time in six months.

Martins appeared to twist his knee near the end of practice at the RFK Stadium auxiliary field. He was helped to the sideline and ice was wrapped around the knee. Team officials, fearing cartilage or ligament damage, took him to a team doctor late yesterday for further examination and scheduled an MRI exam.

Martins, 27, had just been cleared to return to the active roster after tearing his anterior cruciate ligament in December. After months of intense rehabilitation, he had been training with the full squad for several weeks and was getting close to match fitness.

"It's too early to tell" the severity of the injury, United technical director Dave Kasper said. "Is it cartilage, scar tissue? We just don't know right now."

Martins and head trainer Brian Goodstein, who treated the player on the field, were not made available to comment.

"The knee is stable," Coach Peter Nowak said. "Thiago said there was some kind of indication that it was the same spot, but I'm not a doctor. I cannot say right now."

Although Martins hasn't played this season, another extended absence would be a severe blow because Nowak had been counting on him to become a much-needed target player on the front line. For several years, United's attack has lacked a tall, strong forward with scoring abilities. Martins was the MVP of the A-League, the top minor league circuit in the United States, last year before joining United for the final few games of the 2003 season.

In addition to Martins, forward Santino Quaranta and midfielder-forward Eliseo Quintanilla have yet to play this year because of injuries. Several other key players, including all-star defender Ryan Nelsen and midfielders Ben Olsen and Bobby Convey, have missed time with various ailments.

Last Saturday in Chicago, top scorer Jaime Moreno and starting defender Bryan Namoff sat out with injuries. Moreno is expected to play this Saturday against Colorado at RFK, but Namoff was scheduled to see a specialist this week for his abdominal strain.

"For the team, for the mentality, it's not good . . . but I'm never going to complain about injuries because I believe in this team," Nowak said. "We're not going to make excuses."

Nelsen plans to return to the lineup Saturday after missing three games -- one because of a groin injury and two while with New Zealand's national team. But Convey and midfielder Earnie Stewart are with the U.S. national team for the start of World Cup qualifying and will probably miss two MLS matches.

Meantime, Nowak is trying to reenergize United (3-5-3) after last weekend's poor performance, a 3-0 loss to the Fire that could've been much worse.

"The way we play the first seven games, eight games, nine games, we are good in the midfield, we work very hard, we control the game, we play our game, we pressure and we create some chances," he said. "The last game, that wasn't the case in any of these elements, and this concerns me a lot. . . . I'm going to blame myself first. Maybe they weren't prepared mentally for this game."

United Notes: Roland Aguilera, an attacking midfielder from Bolivia who played for Columbus and New England before joining the A-League's Virginia Beach Mariners, began a one- to two-week tryout yesterday. . . . Rookie midfielder Kevin Ara will be in Cambridge, Mass., on Thursday for his graduation from Harvard. He will receive a degree in economics. . . . Freddy Adu's brother, Fro, a 13-year-old defender, helped the under-16 Potomac Cougars win the Maryland State Cup over the weekend.