Cardinals guard Antoine McClain (left) and nose tackle Xavier Williams are on the roster bubble.

The competition for starting spots was minimal heading into Cardinals training camp, and it's only getting rarer.

The projected battles for center and right tackle aren't very close, coach Bruce Arians said Monday, with A.Q. Shipley and D.J. Humphries comfortably ahead. The rest of the offensive starters are pretty much figured out, while the defense only has a couple spots still in flux.

Instead, the next few weeks will be crucial on the back-end of the roster as a glut of capable players push for limited roster space.

"That's where the big competition is," Arians said. "Whether we keep eight, nine or ten linemen will depend how they perform versus a fourth tight end, a fifth running back, a sixth wide receiver."

Guard Antoine McClain took some first-team snaps at right guard in Sunday's practice in place of starter Evan Mathis, who left with leg fatigue. While fans in the crowd may have barely noticed the switch, for McClain, it was a crucial event.

"I'm not wishing injury on anybody, from but my point of view, I was showing the coaches that I could play with that group," McClain said.

The most interesting cutdown may be at defensive line. The Cardinals let go of veterans Matt Shaughnessy and Alameda Ta'amu last season, and will have tough decisions to make again, with nine players having a solid case to be kept.

"Our second is a solid starting unit, and our thirds ain't bad," Arians said. "This is, by far, the deepest defensive line group I've had. It's hellacious competition in that room."

For the high-priced stars, training camp is a slog until the regular season comes. The backups are fighting each day for a job.

"I try not to worry about that, because that's how you get anxiety," McClain said.

PADS SPICE UP PRACTICE

Arians refers to non-padded offseason work as soccer, not football. A few players have termed it the Underwear Olympics. Everyone welcomed Monday's start of padded practices at training camp, and for Arians it allowed the real player evaluations to begin.

It shows "who's physical and who's not," Arians said. "It shows up real fast. Guys who look like All-Americans in shorts fade away. The guys who look like (expletive) in shorts, they're football players. They show up."

Outside linebacker Chandler Jones turned some heads Sunday when he blew up Mathis on a drill and met running back Chris Johnson in the backfield. Arians was thrilled with the intensity.

"That's how we practice," Arians said. "It's full speed."

J.J. NELSON LEAVES PRACTICE EARLY

Wide receiver J.J. Nelson wasn't present for the majority of practice, while wideout Franky Okafor left with a trainer near the end. The position group was already missing John Brown, who was undergoing the NFL's concussion protocol after hitting his head while attempting to catch a pass near the end of practice on Sunday.  

"We're being careful with him," Arians said.

Mathis sat out Monday, but Arians said the leg fatigue following offseason surgery wasn't a big issue. Johnson and quarterback Drew Stanton received a veteran's day off Monday, while cornerback Elie Bouka (hamstring), defensive tackle Robert Nkemdiche (ankle), tight end Jermaine Gresham (hamstring) and cornerback Mike Jenkins (hand) missed practice.

Jenkins had surgery on Monday and could return in two to three weeks, Arians said.