Redskins rookie linebacker Keenan Robinson was undergoing an MRI Friday to determine if he had suffered a torn pectoral muscle, coach Mike Shanahan said.

If that’s the case, then Robinson would be done for the season. He suffered the injury in Thursday’s 38-31 win over Dallas, though it was not discussed after the game. Robinson finished the game.

“It doesn’t look good,” Shanahan said.

Robinson has continued to play more as the season unfolded, providing the Redskins with size (6-foot-3, 240 pounds) and speed. The fourth-round pick helped both rushing the passer and playing in coverage, something he excelled at for the University of Texas. With London Fletcher ailing — he did not finish Thursday’s game because of a sprained ankle — the Redskins don’t want to lose one of his backups. Lorenzo Alexander has filled in for Fletcher as well.

“[Robinson] has a big upside,” Shanahan said. “He’s extremely bright. He gives you everything he’s got. Where he can help us is in passing downs. He’ll be an every-down player for years to come. He has speed and quickness and he’s not afraid to hit.”

The Redskins have had a slew of defensive injuries this season. Linebacker Brian Orakpo, end Adam Carriker and safety Brandon Meriweather, all starters, are out for the season.