Brandon Meriweather (John McDonnell/The Washington Post)

Injured Washington Redskins safety Brandon Meriweather will be sidelined for an estimated four additional weeks after the latest setback to his injured left knee, Coach Mike Shanahan said Monday.

“I’d say it’s going to take Brandon in the area of four weeks before he comes back,” Shanahan said. “That’s just guessing. That’s what our doctors think right now, looking at his knee.”

Meriweather worsened the damage to the medial collateral and posterior cruciate ligaments in his left knee, according to Shanahan, in a collision with wide receiver Aldrick Robinson in pregame warmups before the Redskins’ victory in Tampa on Sept. 30.

 Meriweather has not played all season after signing with the Redskins in the offseason as a free agent. He hurt his knee in the second preseason game, then re-injured it in practice six days before the season opener in New Orleans. He was scheduled to start at safety in the opener in and the Tampa game.

The timetable Shanahan described could keep Meriweather sidelined through the Redskins’ Nov. 11 bye week next month.

Shanahan also said right tackle Jammal Brown might be able to play after the bye. Brown underwent hip surgery and has been on the physically unable to perform list all season. Tyler Polumbus has taken over as the team’s starter at right tackle.

“He just went up to New York and talked to his doctor,” Shanahan said. “The doctor feels very good. He’s going through conditioning. He’s going through football-related drills. The doctors feel like there’s a chance for him to be back after the bye week. So that’s a good sign.”