FOXBOROUGH, Mass. >> Tom Brady’s lawyers demanded that NFL Commissioner Roger Goodell recuse himself from the Super Bowl MVP’s “Deflategate” appeal and threatened to go to court unless the four-game suspension is overturned.

Laying out the grounds for dismissing the penalty and setting the stage for a potential federal court battle, the NFL Players Association said in a letter released Friday that Goodell can’t hear the appeal because he will be called as a witness.

“The NFLPA believes that neither Commissioner Goodell nor anyone with close ties to the NFL can serve as arbitrator in Mr. Brady’s appeal,” the letter said. “If the Commissioner does not appoint such a neutral arbitrator, the NFLPA and Mr. Brady will seek recusal and pursue all available relief to obtain an arbitrator who is not evidently partial.”

Brady was suspended for four games and the New England Patriots were fined $1 million and docked a pair of draft picks after league investigator Ted Wells found that the Super Bowl champions used illegally inflated footballs in the AFC title game.

The team has denied doing anything wrong and published a 20,000-word rebuttal online. Neither the Patriots nor Donald Yee, the agent for their three-time Super Bowl MVP quarterback, responded to requests for comment on Friday.

In their letter, Brady’s lawyers give three arguments for dropping the suspension:

— The evidence collected in the Wells report doesn’t prove Brady violated any NFL rules.

— The punishment is more harsh than for previous, similar violations.

“Indeed,” the union wrote, “no player in the history of the NFL has ever received anything approaching this level of discipline for similar behavior — a change in sanctions squarely forbidden by the CBA and the law of the shop.”

— Under the league’s collective bargaining agreement, only Goodell can punish a player for conduct detrimental to the league. The “Deflategate” penalties were meted out by NFL executive vice president Troy Vincent.

“You have no authority to impose discipline on Mr. Brady under the CBA, and such discipline must therefore be set aside,” the letter said, then adding in a footnote a reference to the Adrian Peterson case: “We also note that one arbitrator has previously found that you, in particular, are unfamiliar with proper NFL discipline procedures and have no role in imposing discipline.”

Brady appealed the suspension Thursday. Late Thursday night, Goodell announced he would hear the appeal personally. Although the collective bargaining agreement gives him the right to do that, the appeal letter claimed Goodell cannot remain impartial because he will called as a witness.

Writing to Vincent, the union said it intends “to call both you and Commissioner Goodell as essential witnesses in the proceeding.” And it told Vincent “your personal involvement in the game-day events surrounding this matter render you inherently biased in any disciplinary determination.”

The team has not said if it will appeal its penalties, which include losing a first-round draft pick next year and a fourth-rounder in 2017. The deadline to appeal is May 21.

Wells concluded that Brady was “at least generally aware” of plans by two team employees to prepare balls to his liking, below the league-mandated minimum. The NFL requires a range of 12.5-13.5 pounds per square inch; Brady has expressed a preference for footballs with less pressure, which can be easier to grip and catch and some quarterbacks prefer those with less air.

The union’s letter comes a day after the Patriots issued a point-by-point rebuttal of the league investigation.

Patriots attorney Daniel Goldberg, who represented the team during the investigation, wrote in the 20,000-word document that those conclusions are “at best, incomplete, incorrect and lack context.”

The response presented its own science to explain the loss of pressure in a more innocuous way. And it claimed that a ballboy’s reference to himself as “The Deflator” was just a reference to losing weight.

It also says increased communication between Brady and the ballboys after the scandal broke were just normal expressions of concern, rather than evidence of the quarterback’s guilt.

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AP sports writers Howard Ulman and Rob Maaddi contributed to this report.