GREEN BAY, Wis. — One day after watching their already-suspect run defense get gashed by Adrian Peterson, the Green Bay Packers found out Monday they now must face life without middle linebacker Nick Barnett.

Barnett is out for the season after tearing a ligament in his right knee in Sunday's loss at Minnesota, leaving Packers coaches scrambling to find a suitable replacement for one of the team's defensive leaders.

"He's very disappointed," Packers coach Mike McCarthy said. "It's unfortunate. But we need to have the other guys step up."

Barnett, a first-round draft pick in 2003, was named a Pro Bowl alternate last season after receiving a contract extension through 2012.

But Barnett didn't make as many big plays in the first half of this season as he did last year, and Packers linebackers coach Winston Moss challenged him to make a bigger impact last week.

Now he won't get that chance.

"Yes, it is disappointing," Moss said. "But Nick is going to be missed. He's in a very unfortunate situation. I'm sure he's heartbroken. Because Nick, I think, gives his all. I mean, he puts everything into it. So not being able to finish out the season like he wanted to is probably going to bother him a lot."

McCarthy said coaches were discussing plans to replace Barnett. They could promote backup Desmond Bishop or move one of their outside linebackers, A.J. Hawk or Brandon Chillar, to the middle — a move made possible by the presence of a third starter-quality outside linebacker, Brady Poppinga.

"We'll probably make that call (Tuesday)," McCarthy said.

Moss didn't tip the Packers' hand, but did note that Hawk has played some snaps at middle linebacker in practice.

"I couldn't put a number on it, but he's been in there before and he's familiar with it," Moss said.

Bishop got the call to fill in for Barnett in the second half of Sunday's game, but coaches weren't exactly thrilled with what they saw.

One play after Barnett was injured while trying to tackle Peterson on the first series of the second half, Vikings running back Chester Taylor turned a short pass into a 47-yard touchdown after Bishop blew the coverage.

Beyond that, Moss said it was Bishop's responsibility to fill the gap that Peterson squeezed through to score the go-ahead touchdown.

"I definitely could've done better," Bishop said after the game. "The touchdown I gave up was bad."

However, Bishop said he was "definitely" comfortable with the possibility of having to take over Barnett's starting role.

Bishop did make a big play by stripping the ball from Peterson on a fourth-and-goal play. And Moss said Bishop might be better prepared after a full week of practice as a starter.

"I think Desmond played very fast, he was very productive, he was impactful," Moss said. "He made a big play on that fourth down. So there were some real positive things that Desmond did with a minimum amount of reps throughout the week. So I feel as though if he's the guy that's going to be in the middle or be on the field, I think with a full week's work, he'll be ready to go."

Either way, the injury is a major blow to a struggling run defense that gave up 192 yards rushing to Peterson.

"Whoever we put in the middle is going to have to be good enough to get it done," Moss said.

Barnett was injured on the Vikings' first drive of the second half while trying to tackle Peterson.

"I shot downhill, fast, trying to get a shot on him, and he cut back," Barnett said after Sunday's game. "So I tried to cut, and it was just bad footing."

Barnett said after Sunday's game he was "just hoping for the best," but McCarthy said medical tests Monday morning revealed the severity of the injury. Surgery hasn't yet been scheduled, and McCarthy did not provide details of Barnett's rehabilitation timeline.

It already had been a turbulent season for Barnett, who forfeited his paycheck for the first game of this season under the NFL's personal-conduct policy after reaching a plea bargain deal for his role in a disturbance at an Appleton nightclub last year.

The NFL also has fined Barnett a total of $15,000 this season for a pair of hits deemed illegal by league officials.

Moss, who played through serious knee injuries during his own career, said rehabilitation will be tough on an intense player like Barnett.

"Initially, he's going to take it pretty hard," Moss said. "But he's going to battle. Nick is going to do whatever it takes to get back on the field as fast as he can. It's a long process. I've gone though it before. I will be there to give him some insight on what he's going to have to do. It's going to be a very, very tough battle. But if anybody can come back as soon as he can, it's going to be Nick."