Redskins running backs Mack Brown (34) and Rob Kelley (20) run a drill during practice on Friday. Kelley enters training camp as the team’s starter. (AP Photo/Steve Helber)

RICHMOND — After squeaking onto last season’s roster as an undrafted free agent and wresting the starting job from Matt Jones in Week 8, Washington Redskins running back Rob Kelley could have arrived at training camp this summer with a swelled head.

But Kelley, 24, who returned to work last week as the incumbent starter in a competitive running back corps that added big-bodied Samaje Perine, a fourth-round pick from Oklahoma, insists his mind-set hasn’t changed.

“Every year I’ve got to make the team,” Kelley said. “This year they draft a running back. Next year they gonna draft a running back. Year after that, they’ll probably draft a running back. You just got to make sure you do your job. You can only control what you can control.”

That’s why he’s approaching each day as if he’s still a fifth-string back, one misstep from being expendable. “It’s very easy to go back down to fifth string,” Kelley said.

It’s wisdom Kelley didn’t have when he survived the last round of roster cuts last season by staging a strong closing case with a 99-yard rushing performance in the final preseason game at Tampa Bay.

“When I first got to the NFL, I was just more happy than anything,” Kelley said. “Then I realized it’s a business. Stuff changes; you got to adapt. You can’t just stay the same. You never stay the same; you either get better or get worse. You got to err on the side of getting better every time.”

In Kelley’s case, he retooled his diet heading into Year 2. Formerly known as “Fat Rob,” he cut out fast food, learned to cook for himself, gained muscle mass while shedding five pounds and, as a result, says he has more energy in practice.

As for the competition from backs trying to take his job, Kelley sees nothing but good in it.

“When Samaje got here, I had to set my game up a notch,” Kelley said. “Even if he’s gonna take this job or not take the job, you’re always going to work on yourself to get ahead of the next person. It’s nothing against Samaje or nobody. In that room, we work hard. We criticize each other. We help each other. And we just feed off each other.

“A lot of guys are hungry. Even if you’re not on a one-year deal, you still feel like you’re on a one-year deal because things can change in the snap of a finger.”

That’s what happened Saturday, when Keith Marshall suffered a season-ending knee injury in the team’s first practice in full pads on a rain-slicked field. Marshall had shown no sign of being hampered by the elbow injury that sidelined him all of last season. But after being brought down by a hard tackle on an impressive carry, he clutched his right knee and was later diagnosed with a torn patella tendon.

It’s possible the Redskins will add another back to compensate, given that Marshall was also eyed as a special-teams contributor.

Fourth-year veteran Chris Thompson, the reliable third-down back and a favorite of Coach Jay Gruden’s, is a lock for making the roster, as is Perine, who rushed for an Oklahoma-record 4,122 rushing yards, along with 49 rushing touchdowns, in his three seasons with the Sooners.

Mack Brown returns. So does former starter Matt Jones, despite his exile to the bench after one fumble too many last season and his request for a trade.

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