Tank Carradine’s knee is sound, but his mind is spinning.

The 49ers defensive tackle has recovered from a torn ACL he sustained in November 2012 that prevented from playing a snap as a rookie and required a second surgery after the season.

Now healthy, Carradine has shown strength, quickness and explosion during training camp. The problem: The 2013 second-round pick hasn’t shown defensive coordinator Vic Fangio a firm grasp on his assignments.

On Sunday, Fangio said Carradine hasn’t yet earned the trust of the coaching staff.

“He was never healthy last year and now he’s getting a chance to learn and show what he can do mentally,” Fangio said. “He’s got a ways to go there yet. Like I’ve said, sitting in those meetings doesn’t mean you’ve learned it and he’s living proof of that. So he’s got to do better from an assignment standpoint for us to feel comfortable to play him. Right now he’s missing too many things mentally.”

Carradine, though, figures to play plenty when he makes his NFL debut Thursday in the preseason opener at Baltimore. Thanks to a rash of injuries, the 49ers figure to have just six defensive linemen available: Carradine, 25, Quinton Dial, 24, Demarcus Dobbs, 26, Tony Jerod-Eddie, 24, Lawrence Okoye, 22, and Mike Purcell, 23.

“We’ll be OK as long as we can keep these guys upright,” Fangio said. “And it’s good work for a lot of those guys, because the guys that are left … they’re all young guys at different stages in their career that need a lot of work and they’re going to get it.”