OXNARD, Calif. (AP) -- David Arkin had never snapped the ball for the Dallas Cowboys before getting to training camp two weeks ago.
When the Cowboys play their preseason opener Monday night at Oakland, Arkin could be snapping for quarterback Tony Romo.
Starting center Phil Costa missed practice Saturday and got an MRI exam because of a lower back injury that forced him out of practice the previous day.
Cowboys owner Jerry Jones said after practice that the MRI results were good with no major damage, and that Costa wasn't being completely being ruled out of the Raiders game.
Receiver Dez Bryant left practice early Saturday because of hamstring tightness, though Jones said the team was just being cautious. Bryant could be iffy against the Raiders if the tightness doesn't subside.
Rookie cornerback Morris Claiborne returned to the padded practice Saturday after the sixth overall pick missed a week with a sprained left knee. He isn't expected to play against the Raiders.
"I thought he moved good. Sound again," Jones said. "I don't know that you will see him at all on Monday, but it was good to see him back out there."
Arkin, a fourth-round draft pick in 2011 who didn't play any last season for the Cowboys, worked the first-team offense Saturday.
The Cowboys have been decimated by injuries in the interior line. Kevin Kowalski and Bill Nagy, the other players who had been expected to compete with Costa at center, were already out because of ankle injuries.
"We will continue to evaluate that situation as we go forward and just try to give ourselves the best chance inside," coach Jason Garrett said. "We've had a rash of injuries at that spot, so we are just trying to manage it the best we can."
Arkin started 44 games at Missouri State, mostly at right guard with a few games at left tackle.
Mackenzy Bernadeau, the veteran free-agent guard added this offseason, is also a potential center. But Bernadeau missed all offseason workouts because of hip and knee surgeries, and didn't have his first full padded practice until Thursday. The Cowboys are trying to get him back in the flow of playing at the guard spot where is most comfortable.
"We want to keep Mackenzy at guard as much as we can," Garrett said. "One of the things we like about him was his position flex to play center. But he has missed so much practice. We are really focused on him playing guard. He certainly could be an emergency go for us."
Garrett said the Cowboys have "cross-trained" a number of guys in camp for center duty.
"We just want to make sure they are game ready," Garrett said.
