The NFL Players Association filed a legal brief saying Commissioner Roger Goodell's requirement that Adrian Peterson attend counseling was unlawful, reports ESPN.com. The union claims Goodell's disciplinary powers don't extend past suspensions, fines and contract terminations to counseling, the report said.

In November, Goodell suspended Peterson without pay for the rest of the season and said the Vikings running back cannot be reinstated until at least April 15. The move came after Peterson pleaded no contest to recklessly injuring his 4-year-old son last May.

In his letter to Peterson, Goodell set several requirements for reinstatement, including counseling.

"In order to assess your progress going forward, I will establish periodic reviews, the first of which will be on or about April 15, 2015," the commisssioner wrote. "At that time, I will meet with you and your representatives and the NFLPA to review the extent to which you have complied with your program of counseling and therapy and both made and lived up to an affirmative commitment to change such that this conduct will not occur again. A failure to cooperate and follow your plan will result in a lengthier suspension without pay."

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