Linebacker Keenan Robinson, whose 2012 season ended after a torn pectoral muscle, is out again this season. (Matt Strasen/Associated Press)

RICHMOND – The Redskins have likely lost second-year inside linebacker Keenan Robinson to a torn left pectoral muscle just one day into training camp.

A fourth-round pick out of Texas last season, Robinson is expected to be sidelined three to five months, a person with knowledge of the situation said Friday morning. NFL Network first reported the extent of Robinson’s injury.

Robinson left Thursday afternoon’s practice with an injury, and had an MRI exam done on his left shoulder area, Coach Mike Shanahan said at the time.

This will mark the second straight season that Robinson has suffered such an injury. Eleven games into his rookie season, Robinson tore his right pectoral muscle and missed the remainder of the year. He spent the offseason working his way back from that surgery, and didn’t take part in any of the offseason practices.

Robinson received full clearance for training camp, but didn’t make it through a full practice before tearing the pectoral muscle on the opposite side.

The Redskins had planned for Robinson to compete for a backup spot to starting inside linebackers London Fletcher and Perry Riley.

Related: Redskins place Robinson on IR, sign Muckelroy (Nov. 2012)

Have a Redskins question? E-mail Mike Jones at with the subject line “Mailbag question” for him to answer it in The Mailbag.

Follow: @MikeJonesWaPo | @MarkMaske | @john_keim @Insider | @PostSports

What’s ahead:

The team will practice today at 10 a.m. and 3:20 p.m.

More from The Post on the NFL and Redskins:

Observations from Day One of training camp

DL Jenkins in spotlight with Carriker out

Robert Griffin III takes it slow as Redskins open training camp

Reid: Offense will pave Redskins’ way to sustained success

Training camp photo gallery: 2013 season gets underway

Fred Davis expects slightly reduced camp workload

Mazda is refurbishing Alfred Morris’s beloved ’91 626

Rookies Amerson, Rambo open camp as first stringers

Orakpo says contract situation will ‘work itself out’

D.C. Sports Bog on the Redskins

Our complete guide to training camp in Richmond