Updated

Washington, DC (SportsNetwork.com) - As expected, the NFL Players Association has filed an appeal of Commissioner Roger Goodell's suspension of Minnesota Vikings running back Adrian Peterson for violation of the league's personal conduct policy.

In a letter sent to the Commissioner's office, the union accused Goodell and the NFL of denying Peterson due process by "making up the process and punishment as it goes" while calling for the case to be heard by an independent and neutral arbitrator.

Goodell announced Tuesday that Peterson will be suspended indefinitely without pay -- and at least through the remainder of this season -- after pleading no contest to a charge of misdemeanor reckless assault for allegedly striking his 4-year-old son with a wooden tree branch last May.

Peterson will not be considered for reinstatement prior to April 15 under the recent ruling.

In addition to questioning Goodell's impartiality over the case, the union contested several issues with the verdict that it viewed as inconsistent with previous NFL decisions regarding the personal conduct policy.

It pointed out that no first-time offenders have served more than a two-game suspension for acts of domestic violence, with the currently on-appeal Ray Rice case the lone exception. The union also argued that since Peterson's incident occurred in May, it should not fall under the revised personal conduct policy that the league issued in late August.

That change came as a response to a firestorm of public criticism Goodell received for initially handing the ex-Baltimore Ravens running back Rice a two-game ban for physically assaulting his future wife in an Atlantic City casino. The Commissioner later admitted he "didn't get in right" in his handling of the Rice situation.

The union also claimed that league officials told NFLPA senior vice president Troy Vincent that Peterson would be credited for time served during the period he spent on the Commissioner's Exempt list while his court case was ongoing.

Peterson has not played since the Vikings' season opener on Sept. 7, though he had been receiving his full salary while on the exempt list until Tuesday's ruling.

The NFLPA closed the letter by demanding that Goodell recuse himself from the appeals process, saying "there is simply no way for you to impartially arbitrate Mr. Peterson's appeal."

"Given the intense criticism that you and the League office have faced from NFL business partners, Congress, the media, NFL fans and the public at large, and the public pronouncements and commitments you have personally made in response to this criticism, it is clear that you have, by your actions, rendered yourself evidently partial and biased in this matter," the letter read.

Peterson was originally charged with reckless or negligent injury to a child, but pleaded no contest to the lesser offense of misdemeanor reckless assault after his legal team worked out an agreement with the Montgomery County (Texas) district attorney. The 2012 NFL MVP avoided jail time and was placed on probation while receiving a $4,000 fine and an order to perform 80 hours of community service.