Source: Redskins' Guice (knee) out 6-8 weeks

ASHBURN, Va. -- Washington Redskins running back Derrius Guice underwent knee surgery to repair his right meniscus and will miss six to eight weeks, a source told ESPN's Adam Schefter on Thursday.

The injury creates an opening for Adrian Peterson to start just one week after he was a healthy inactive for the season opener.

Guice tore his meniscus in Sunday's 32-27 loss to the Philadelphia Eagles. The Redskins initially hoped he would miss only a couple of weeks, but the knee didn't show any improvement in the days after the game.

He was then examined by Dr. James Andrews, who performed the surgery.

On Wednesday, coach Jay Gruden said Peterson would start against the Dallas Cowboys if Guice needed to miss time.

Peterson said he's gone through a "roller coaster" of emotions over the last week.

"You're at an all-time high leading up and then you're at an all-time low, not really understanding and climbing and getting back up here," Peterson said Thursday. "I'm a professional. I've been through way worse than not being [active] for Week 1 through my life.

"Sunday being out there was tough, and I was in a totally different mindset. But ... it's not like it was three or four weeks where I was doing practice-squad things and not really being involved. It was just a week off, pretty much."

Peterson didn't want to say whether or not he remained angry at Gruden over last week's decision.

"I don't want to continue to talk about it because it's Thursday," Peterson said. "We got a game on Sunday [against] the Cowboys. We're moving on. For me, it's water under the bridge."

In two games against the Cowboys last season, Peterson rushed 24 times for 99 yards in a win and 12 times for 35 yards in a loss.

Peterson, who has a base salary of $1.03 million, can earn another $1.5 million in incentives -- $500,000 if he rushes for more than 1,000 yards; $500,000 if he reaches that mark and the Redskins make the playoffs; $250,000 for 10 rushing touchdowns; and $250,000 with that many TDs and a postseason berth.

He said he texted with Guice, offering words of encouragement and telling him to focus immediately on his recovery.

Gruden also had positive words about Guice's ability to recover.

"I don't have a concern with him getting back to full strength," Gruden said of Guice's recovery. "He came back extremely strong with the much more damaged knee last time. This is a minor procedure from what I've been told and he will be back. Great kid, works extremely hard and eventually he's going to show Redskin Nation what he's all about."

Guice has now suffered a knee injury in two of the three games he has played for Washington; he tore his left ACL in the 2018 preseason opener. This summer, he made it through the third preseason game with no issues.

In college, he hyperextended his right knee while playing for LSU.

Guice carried the ball 10 times for 18 yards against the Eagles on Sunday.

Gruden deactivated Peterson in the opener, the first time he had been a healthy inactive in his 13-year career. The Redskins have typically kept only three running backs active. Because they planned to just use Guice and third-down back Chris Thompson, Gruden wanted the third back on special teams -- which is why Wendell Smallwood was active over Peterson.

But the move surprised numerous players.

"You don't have too many walking Hall of Famers on your football team," right tackle Morgan Moses said after the game. "I commend him because it's probably harder on him than me being able to deal with sitting on the sideline, but obviously we think he's a hell of a football player and has a lot of juice left. Hopefully, you know, it's just a one-week thing and we can move forward."

Peterson started 16 games for Washington last season, rushing for 1,042 yards and seven touchdowns. He is eighth on the all-time rushing list with 13,318 yards and tied for fifth with 106 rushing touchdowns.

After Sunday's loss, when asked further on why he made Peterson inactive, Gruden said, "If we have a game where we think we can run the ball 55 times in a game in I-formation, then sure, I'll get him up."

But on Wednesday, Gruden said, "Adrian is a pro, and I feel very good, if his number is called this week, he'll be ready to go and play very hard. He'll be Adrian Peterson."