WASHINGTON (Reuters) - Minnesota Vikings star running back Adrian Peterson was suspended by the National Football League without pay for at least the remainder of the 2014 season on Tuesday for committing an "incident of abusive discipline" on his 4-year-old son. The action taken by Commissioner Roger Goodell against one of the NFL's marquee performers comes as America's most popular sports league struggles to contain a scandal sparked by domestic violence incidents involving several players. The NFL players union quickly condemned Goodell's action as "inconsistent," "arbitrary" and evidence of a "credibility gap" for the league. It said it will appeal the suspension and demand that a neutral arbitrator oversee the matter. An independent arbitrator heard the case of Ray Rice, who Goodell suspended from the league indefinitely after a video showed the former Baltimore Ravens star knocking his fiancée unconscious in an elevator. Peterson, 29, pleaded no contest in Conroe, Texas this month to a misdemeanor assault charge and was ordered to pay a $4,000 fine and perform community service. Images showing Peterson's son with cuts and bruises all over his body including his buttocks and genitals provoked outrage that tarnished the reputation of one of the most productive running backs of his generation and added to the league's public relations nightmare. In a sharply worded letter to Peterson, Goodell said the Vikings star had "shown no meaningful remorse" for his conduct, noting that a Texas grand jury concluded Peterson had "overstepped the bounds of acceptable corporal punishment and engaged in physical abuse of your child." "While an adult may have a number of options when confronted with abuse - to flee, to fight back or to seek help from law enforcement - none of those options is realistically available to a 4-year-old child," Goodell wrote. Peterson will not be considered for reinstatement before April 15 for violating the NFL Personal Conduct Policy, the league said. He was arrested in September on a felony child abuse charge for disciplining his son by repeatedly striking him with a thin tree branch called a switch. Goodell said Peterson must undergo intensive counseling and treatment before being considered for reinstatement. An NFL statement called Peterson's action an "incident of abusive discipline" and warned against any "repetition of this conduct." Goodell wrote that "the injury inflicted on your son includes the emotional and psychological trauma to a young child who suffers criminal physical abuse at the hands of his father." Goodell added that "the repetitive use of a switch in this instance is the functional equivalent of a weapon, particularly in the hands of someone with the strength of an accomplished professional athlete." The union said Peterson was told that after his legal case was resolved he would be allowed to play again. Goodell disputed that, saying Peterson was always subject to NFL discipline regardless of the outcome of his legal case. Peterson played in the season's first game before being placed on Goodell's "exempt" list, keeping him off the field but allowing him to draw his $11.75 million salary.