King was on hand for last week's organized team activities and watched the former LSU star make a diving deflection of a Carson Palmer pass. Mathieu was "buzzing around plays" during workouts. If these early indications mean anything, he's more than a supporting cast member.
"He reminds (me) of Troy Polamalu with his closing speed," Palmer told King. "He might not be the fastest guy out there, but he can change directions and get to the ball really fast, like Troy."
Off the field, NFL.com's Ian Rapoport reported on May 23 that Mathieu -- who is under contract with the Cards -- already has entered the NFL's drug-testing program and might face up to 10 tests per month.
The 5-foot-9 Mathieu doesn't have prototypical size at safety, but he's a tantalizing option as an extra defensive back. In today's NFL, nickel corners play like starters. That might be less true in the run-heavy NFC West -- where the 49ers and Seahawks love their double-tight end sets -- but Mathieu will have an early opportunity to make an impact.
Follow Marc Sessler on Twitter @MarcSesslerNFL.