COSTA MESA, Calif. – Like all head coaches, Anthony Lynn is more concerned with the players on the field than in the training room.

Lynn only saw Mike Williams, this year’s first-round selection, practice for about a half-hour in May before the receiver suffered a lower-back disk herniation that would keep him out of the team's entire offseason program. This weekend, the Los Angeles Chargers placed Williams on the active/physically unable to perform list and said the Clemson product will not practice for the rest of training camp as he rehabs from the injury.

"He's doing well in rehab, and when he's healthy, he's healthy," Lynn said. "We can't control that."

I asked Lynn if he was concerned that Williams could be out the duration of his rookie season. The coach did not provide the most optimistic response.

"I'm hopeful that it's not, but who knows?" Lynn said. "It could be. I don't know. You'd like to have him out [there], but fortunately we have depth at that position and we're going to be OK."

Williams was out on the field most of the practice on Sunday, which appears to be a good sign, and caught some balls off the JUGS machine. He's also done some light running. The Chargers remain hopeful that Williams' back injury calms down enough so that he can return to the field this season.

Barner, Cumberland flash: Two players I thought showed some explosiveness on offense were running back Kenjon Barner and tight end Jeff Cumberland. Both burst out in the open field during 7-on-7 and team drills. Barner signed with the Chargers as a free agent and is competing for a job as a change-of-pace back to complement Melvin Gordon. Cumberland suffered an Achilles tear during preseason play last year, but it looks like he's healed and will compete for a job among a deep group at tight end. The Chargers used a lot of heavy formations on Sunday, with two tight ends and two running backs. It could have been just a focus for the practice session, but that's something I'll keep an eye on in future practices.

Punt return job up for grabs: Travis Benjamin, Desmond King, Artavis Scott and Mitchell Page took reps at punt returner during special-teams period. I was particularly impressed with how comfortable Page looked back there.

Injury update: Along with Williams, cornerback Jason Verrett (knee) and safety Darrell Stuckey (knee) did not practice and remain on the PUP list. Players who did not practice during offseason work but returned to the field include defensive lineman Caraun Reid (knee), offensive lineman Donavon Clark (knee), cornerback Casey Hayward (ankle) and edge rusher Chris McCain (ankle). With Verrett out, Craig Mager worked with the first group at cornerback.