Cardinals running back Adrian Peterson breaks off a run against Jacksonville last month.

Bruce Arians said this week he had "no idea" if Adrian Peterson was going to be able to play again this season.

The coach – and the Cardinals – have a pretty good idea now.

After Thursday night reports that the veteran running back would be placed on injured reserve because of the neck injury he suffered against Jacksonville, Arians said Friday Peterson would be going on IR. Since the injury, Peterson, 32, has missed two games and has not practiced.

Friday afternoon, the Cardinals finally put Peterson on IR officially. The spot was needed after the Cardinals claimed cornerback C.J. Goodwin off waivers, following Goodwin's release from the Falcons. Goodwin, 27, played in 26 games for Atlanta in the secondary the past two seasons after transitioning from wide receiver.

Friday morning, Peterson had already confirmed his fate with an Instagram post.

"The good news is my neck injury doesn't require surgery, but I'm told by Dr. Robert Watkins that the best thing for me is just more rest than the length of the season," Peterson wrote. "Obviously I'm disappointed about going on IR when my body is still able to produce, but I look forward to returning completely healthy for another season in 2018. I'm grateful for the opportunity that the Arizona Cardinals have given me to show I can still contribute on an NFL team."

A return to the field was already sounding doubtful. Asked multiple times about

the injury since Peterson got hurt, Arians only said Peterson needed rest and that any return would be up to Peterson's doctors.

Peterson has not spoken locally to the media since the injury, but posted a video of the play last week on Instagram, writing in part "What a blessing. I could've easily broken my neck or seriously injured my neck. No surgery required, just some TLC & Rest!" 

Kerwynn Williams has been the starter in Peterson's absence and he was already slated to do that again Sunday in Washington. Since Peterson went down, Williams has gained 170 yards rushing on 36 carries in two games.

Peterson, acquired in a trade with New Orleans in October, played in six games for the Cardinals, rushing for 448 yards and two touchdowns on 129 carries, a 3.5-per carry average. He had a pair of 100-yard games, rushing for 134 yards in his debut against Tampa Bay and 159 – on a career-high 37 carries – at San Francisco. It was the most carries in NFL history for any player at least 30 years old.

His final appearance against the Jaguars, Peterson totaled 79 yards rushing on 20 carries.

Including four games with the Saints, Peterson gained 529 yards on 156 attempts and two touchdowns.

Peterson is under contract through 2018, with compensation next season worth up to $3.5 million with salary, roster bonuses and workout bonuses. The Cardinals also have starter David Johnson coming back next season.

Asked if Peterson could return to the team in 2018, Arians said "no doubt."

"He'll heal fine," Arians said, adding that he thought there could be enough work for both Johnson and Peterson.

"David, he learned his lesson, he doesn't need it every play," Arians said.

But Arians didn't want to get into exactly how he might be able to deploy Johnson and Peterson at the same time.

"I don't even think about next year," Arians said, chuckling. "I have enough problems with Washington this week."