Athletic trainers Tom Reed (in visor) and Chad Cook look at the left calf of defensive tackle Corey Peters Wednesday while fellow defensvie linemen Frostee Rucker (92) and Cory Redding (90) look on. Peters was declared out for the season Thursday with a ruptured Achilles.

Corey Peters had been playing very well for the Cardinals in a role that he felt had almost been designed just for him. But the nose tackle isn't going to get a chance to show it, after he ruptured his left Achilles in practice, ending his season.

Coach Bruce Arians acknowledged the bad news Thursday, before saying that fourth-round draft pick Rodney Gunter – who has been improvingly significantly as camp progresses – will take Peters' spot in the starting lineup.

"Time for the rookie to step up," Gunter said. "Time to go to work. Time to grow up fast."

The Peters' news wasn't the only injury hit the Cardinals took. Arians said starting left guard Mike Iupati will have surgery to repair the meniscus in his left knee, sidelining him anywhere from 3-to-8 weeks. Ted Larsen will move into the lineup, although Earl Watford will also have a chance to win the job, Arians said.

But Iupati will be back. Peters, like Iupati one of the Cardinals' big free-agent signings, will not. The injury comes after Peters ruptured his right Achilles at the end of the 2013 season, making his 2014 season less than he wanted.

Linebacker Sean Weatherspoon, who was teammates with Peters in Atlanta before both signed as free agents this March, had a chance to spend time with Peters Wednesday night after Peters knew he was done. For Weatherspoon, Peters is in a different place than 2013.

"To see him then and to see him now, I feel a lot better about where he is mentally," Weatherspoon said.

The Cardinals take a gut punch emotionally to lose a player who is well liked and starts on the field. But this is the

reason General Manager Steve Keim built so much depth on the defensive line. There were going to be NFL-type players who weren't going to make the 53-man roster. Now the Cardinals – who have undrafted rookie Xavier Williams and Alameda Ta'amu at nose tackle – can make it work without having to sign anyone.

"The NFL is not if you're going to get hurt – you're going to get hurt," defensive end Cory Redding said. "We lost a brother and a teammate, but we've got to keep going. The young boys have got to step up. Some guys have to grow up really quick."

That starts with Gunter, whom the Cardinals loved so much as a raw small-school prospect they traded up to get in the draft.

"I'm surrounded by greatness," Gunter said. "I have no other choice but to be great."

Cornerback Jerraud Powers said Gunter is "one of those country guys you wouldn't want to fight at a bar fight."

"He's quiet, doesn't say much, but he walks around and he has that look," Powers added. "We've been riding those

little scooters around and he stood on mine one time as I was walking in the hall. I just acted like it wasn't even mine. I just let him have it."

Gunter just started getting some nose tackle reps last week in practice and looked good doing it, defensive end Calais Campbell said.

"Funny how things work that way sometimes," Campbell said, noting that in the end, the Cardinals will replace Peters "by committee."

Redding likes the idea the Cards won't just turn to the waiver wire. The Cardinals went through this just about this time last season, when Darnell Dockett was lost for the season. At that point, the Cards brought in Tommy Kelly. This time, they have the players to absorb such a blow.

It's the brutal part of the game, but Arians' message to the team was simple.

"You tell them you feel bad for Corey," Arians said, "and practice today at 2 o'clock."