Buffalo Bills defensive coordinator Mike Pettine isn’t eager to cast the first stone when it comes to the team’s failings on that side of the ball last season prior to his arrival.

But it’s clear he saw what anyone watching the Bills did: An extreme makeover was needed.

Pettine is not Ty Pennington, but he’s hard at work rebuilding. Asked how he sees the unit coming together, the words “aggressive” and “relentless” are the first out of his mouth.

“It’s an entire change of mentality here. Not just from a scheme standpoint, but from a fundamental standpoint, from a mentality standpoint, so we’re knee deep in trying to get that taught,” Pettine said this week in his second public comments since being hired by Doug Marrone in January. “It’s all new, and I think they’ve done a good job of buying in. They’ve been very eager to learn, and they’ve carried it over onto the field.

“I wasn’t here, I don’t want to get too much into last year, I just know the system that we’re installing takes advantage of what our guys do well, and like I said, they’ve been very accepting of it. Now, again, I think they’re all feeling pretty good about it. We haven’t given up a first down, a yard or a point.”

They’ll keep that perfect record intact until Sept. 8, when the New England Patriots visit Ralph Wilson Stadium for the season opener. Given that 108 long summer days must pass between now and then, it’s not surprising that Pettine is not overly concerned right now with what his depth chart looks like at any position.

“We’re rotating guys and different personnel groupings and we just want to find out who can play. So we’re a long way from trying to hone it down and trying to figure out who’s going to be out there,” he said. “That’s to me a little bit too far down the road to talk about.”

Pettine stays very much in the moment. That explains why he didn’t seem bothered at all that he hasn’t met, or even spoken with, Pro Bowl safety Jairus Byrd. Byrd, who was given the franchise tag by the team, has yet to sign it or work out a long-term contract, and is staying away from the voluntary practices.

“It’s a coaching cliche, but I can only really deal with the players that are here. I haven’t met him, and again, he’s not here,” Pettine said. “For some of the other guys that aren’t here, we have plenty to do with the guys that are. We really haven’t, as a staff, given it much thought.”

Pettine, however, did share his thoughts on a few players through the first five of 10 organized team activities.

On rookie linebacker Kiko Alonso out of Oregon: “Kiko’s had a good start for us. To me, it’s a credit to the coaching staff at Oregon. I mean, he’s NFL ready. He understands a lot of the concepts we’re playing. They played a lot of them in college.”

Pettine, though, cautioned that not too much should be read into Alonso starting team drills with the starting unit this week.

“It’s a credit to his preparation. He came in and did a good job in the rookie camp. Now, we’re still in the process of rolling guys through. We’ve been alternating. This was his week to get with the ones,” he said. “It’s still a work in progress. There’s been no commitment made there.”

On Aaron Williams, who’s making the transition from cornerback to safety: “He’s doing a nice job, and that’s a difficult thing. You could make the argument, ‘well he’s a DB and he’s staying a DB.’ The safety world is very different from the corner world. He’s very intelligent and he’s soaking it all in. Every day is a challenge for him as far as new concepts in the playbook and I think he’s done a real nice job buying in and really getting quickly to the graduate-level details of it.”

Pettine again reiterated his desire to have a flexible defense. Defenders like Mario Williams and Bryan Scott have said this offseason, only half-jokingly, that they’re not even sure what position they’re playing sometimes in the scheme.

“I think a cornerstone of this is flexibility,” Pettine said. “You have to have the ability to have guys play multiple positions and play multiple jobs. If you want to confuse an offense, I think you have to have guys that can be a defensive end and outside linebacker, a safety and inside linebacker, a nickel and outside corner or safety.

“A big part of the NFL is identifying guys pre-snap. We want to limit the amount of pre-snap information we’re putting out to an offense. That’s why we look for guys that have that type of versatility, and then at the same time you’re cross-training guys to play other positions.

“Injuries are a part of the game. We’re always in the business as a coaching staff to put our best 11 out there. And we want to be in a position to do that, so when you have guys that know multiple jobs, that makes it a lot easier.”