Cardinals safety Tyrann Mathieu (right) with wide receiver Larry Fitzgerald prior to Friday's preseason game against the Raiders

SAN DIEGO – Tyrann Mathieu may not be practicing yet, but he's one step closer, and he made all of his teammates aware of that fact Tuesday morning.

The Cardinals' All-Pro safety was activated from the physically unable to perform list along with cornerback Justin Bethel (foot) and Frostee Rucker (ankle) before the team's walkthrough. While Mathieu didn't do any running, cutting or tackling, he was lively in his first day back on the field since tearing his ACL near the end of last season.

"You could kind of notice that he was coming off that PUP list," safety Tony Jefferson said. "He started talking a lot more in meetings, being more involved. He's hyped up and excited. We went through walkthrough today and he's jumping around. I'm like, 'Calm down. In walkthrough we don't do nothing.' But he's excited, and I'm excited to see him back. Even as his teammate, you just enjoy watching him play."

Coach Bruce Arians cautioned that the move doesn't mean a return to practice is imminent for any of the three, or that they are on target for the regular season opener against the Patriots on Sept. 11.

"I wouldn't say that," Arians said. "I'd just say that they're on target to get mental reps. They look good. Ty looks real good right now. Justin still favors (his foot) a little bit, but he's fine. Frostee's got to get into better shape."

Arians was optimistic about Mathieu's progress, though. A knee brace hindered his effectiveness in 2014 as he returned from a torn ACL, but the team has found him a more comfortable one to wear. He did some rehabilitation work in Arizona on Monday and the reports back were encouraging.

"We have the computers on him which tells the strength in each leg," Arians said. "He's real, real close."

Mathieu wasn't available to reporters but took an excited tone on Twitter, contrasting this season to his subpar 2014 campaign coming off the other major knee injury.

"But this time it won't take me (too) long to get back in my groove..... If I told you my goals this year you'd count me out," he tweeted.

NIKLAS GETS FIRST-TEAM WORK

Tight end Troy Niklas' ascension continues. The former second-round pick was hampered by injuries in each of his first two seasons, but he's been healthy throughout training camp and Arians said he would get first-team work in Tuesday's joint practice with the Chargers.

A back injury to Jermaine Gresham left an opening, and based on Arians' regular praise over the past few days, Niklas has done plenty to deserve the nod. Arians has always maintained it was a matter of staying healthy for Niklas, who injured his hamstring days before training camp last season and has had a variety of other ailments.

"He kept overtraining and getting hurt," Arians said. "He finally relaxed and just did what (strength and conditioning coaches) Buddy (Morris) told him to do, and Roger (Kingdom) told him to do. He's reaping the benefits."

Niklas had two catches for 44 yards in the preseason opener against the Raiders and had some nice blocks in the running game.

"Coming in this year, I felt like I actually had a fighting chance," Niklas said. "That's what's made it more fun, more of a competition. When you come in injured, you know you're not 100 percent and it's not very fun."

JOHN BROWN STILL IN CONCUSSION PROTOCOL

Wide receiver John Brown didn't make the trip to San Diego as he remains in the concussion protocol after a hit to the head early in training camp.

"There's always a concern when there's a concussion, but we're going to take our time until he's totally cleared," Arians said. "And then take a few more days, because he's got to get back in shape now. He's going to be totally out of shape."

Defensive tackle Robert Nkemdiche (ankle) is expected to make his training camp debut next week, Arians said. Defensive tackle Ed Stinson injured a muscle against the Raiders and is day-to-day, while guard Earl Watford is continuing to progress from a knee bruise, Arians said.