"The NFLPA argues that recusal is required because it believes that I may be a 'necessary' and/or 'central' witness in the appeal proceeding," Goodell wrote. "I have carefully considered this argument and reject its premise. I am not a necessary or even an appropriate witness, much less a 'central witness' as the NFLPA contends."
He added: "There is no basis upon which I could properly be asked to testify in the appeal proceeding, which under Article 46 of the [collective bargaining agreement] is designed to afford Mr. Brady an opportunity to bring new or additional facts or circumstances to my attention for consideration."
Brady's suspension was announced May 11, days after independent investigator Ted Wells released his report saying the Patriots quarterback was "at least generally aware" that locker room staff were deflating balls for the 2014 AFC championship game.
The NFLPA filed an appeal on Brady's behalf three days later.
"I very much look forward to hearing from Mr. Brady and to considering any new information or evidence that he may bring to my attention," Goodell wrote in his letter to the NFLPA. "My mind is open; there has been no 'prejudgment' and no bias that warrants recusal."
He added: "Because protecting the integrity of the game is the commissioner's most important responsibility, I decline to rewrite our collective bargaining agreement to abrogate my authority and 'discretion' to hear 'any appeal'" in a case that involves allegations of conduct detrimental to the integrity of the game.