Rookie Keenan Robinson suffered a pectoral muscle injury on Thursday. (Matt Strasen/Associated Press)

The Washington Redskins have placed rookie inside linebacker Keenan Robinson on Injured Reserve, and have filled his roster spot with third-year pro Roddrick Muckelroy a team source confirmed Monday.

Robinson, Washington’s fourth-round pick (119th overall) out of Texas, suffered a torn pectoral muscle late in last Thursday’s game at Dallas.

Redskins coach Mike Shanahan said Friday that the rookie was undergoing tests to determine the extent of his injury, but added, “right now, it doesn’t look good.” The coach said that the team feared Robinson would be lost for the season with the same injury that forced fellow linebacker Brian Orakpo out for the year in Week 2.

The 6-foot-3, 238-pound Robinson had been used primarily to spell 15-year veteran London Fletcher, but had played more in the last two weeks with Fletcher hobbled by a sprained left ankle. Robinson played 19 defensive snaps and 21 special teams snapsin Thursday’s game at Dallas.

The team signed another Texas product to replace Robinson. Muckelroy was on the practice field Monday as the Redskins began preparation for next week’s game against the New York Giants.

The 6-2, 250-pound Muckelroy was drafted in the fourth round (131st overall) by Cincinnati in 2010. He appeared in 14 games as a rookie, recording 10 tackles, and then missed all of last season with a ruptured Achilles’ tendon. Muckelroy played in five games this season – his last game was in Week 9 against Denver – before he was released by Cincinnati.