Adrian Peterson has been suspended without pay for at least the remainder of this season.

The NFL announced its highly anticipated ruling Tuesday morning, stating that the Minnesota Vikings' star running back will not be considered for reinstatement before April 15, 2015, for violating the league's personal conduct policy.

The league said it informed Peterson of the ruling in a letter from commissioner Roger Goodell, who spelled out a path for his return to the field.

The Vikings have six games left this season. The league's enhanced disciplinary policy calls for a six-game suspension for first offenses of assault, battery or domestic violence. Aggravating circumstances warrant higher levels of discipline, and Goodell's letter to Peterson spelled that out.

But a league source told ESPN's Ed Werder that it is "very possible" that Peterson could return to the field for this Sunday's game against the Green Bay Packers, pending a ruling by the independent arbitrator in Monday's grievance hearing.

The NFL Players Association quickly announced its plan to appeal and sharply rebuked what it calls the league's inconsistency and unfairness in the process.

If arbitrator Shyam Das rules in favor of Peterson, he could play while his appeal of Tuesday's suspension is heard, according to rules laid out in the NFL's collective bargaining agreement. Goodell, under the CBA, would decide who hears Peterson's appeal.

Another source told ESPN's Chris Mortensen that if Das decides Peterson should be reinstated, the issue will become more complex and "there will be many lawyers involved."

"You have shown no meaningful remorse for your conduct. When indicted, you acknowledged what you did but said that you would not 'eliminate whooping my kids' and defended your conduct in numerous published text messages to the child's mother. You also said that you felt 'very confident with my actions because I know my intent.' These comments raise the serious concern that you do not fully appreciate the seriousness of your conduct, or even worse, that you may feel free to engage in similar conduct in the future."

Peterson was indicted in September on a felony charge of injury to a child for using a wooden switch to discipline his 4-year-old son earlier this year. The All-Pro pleaded no contest to a lesser charge of misdemeanor reckless assault Nov. 4, setting the stage for the NFL's ruling.

Peterson was placed on a special exempt list at the sole discretion of Goodell on Sept. 18, essentially paid leave while his case went through the legal system.

The NFLPA said Peterson was told that would count as time served for any suspension levied, citing an unnamed NFL executive. League spokesman Brian McCarthy told The Associated Press in an email that Peterson's stay on the exempt list was taken into account.

"There were aggravating circumstances that led to the discipline announced," McCarthy told the AP.

The league's statement included excerpts of the letter written by Goodell, who has required that Peterson undergo counseling and treatment in order to be reinstated.

"We are prepared to put in place a program that can help you to succeed, but no program can succeed without your genuine and continuing engagement," Goodell wrote in the letter. "You must commit yourself to your counseling and rehabilitative effort, properly care for your children, and have no further violations of law or league policy."

The Vikings also released a statement, saying they "respect the league's decision and will have no further comment at this time."

Peterson's case revived a debate about corporal punishment, which is on the decline in the U.S. but still widely practiced in homes and schools. Peterson has repeatedly claimed that he never intended to harm his son and was disciplining him in the same way he had been as a child growing up in East Texas.

Goodell, however, expressed concern in his letter that Peterson does not "fully appreciate the seriousness" of his conduct.

"You have shown no meaningful remorse for your conduct," Goodell's letter said. "When indicted, you acknowledged what you did but said that you would not 'eliminate whooping my kids' and defended your conduct in numerous published text messages to the child's mother. You also said that you felt 'very confident with my actions because I know my intent.'

"These comments raise the serious concern that you do not fully appreciate the seriousness of your conduct, or even worse, that you may feel free to engage in similar conduct in the future."