In a series of posts on Instagram, Brady said the rule change will cause issues for offenses when attempting to identify blitzers or the middle linebacker of opposing defenses.
"Good luck trying to block the right people now!" Brady posted on his Instagram story. "Going to make for a lot of bad football!"
Brady then posted a screenshot of a story about the new rule and directed it to the NFL and the NFL Players Association, writing: "Why not let the linemen wear whatever they want to? Why have numbers? Just have colored jerseys...why not wear the same number?...DUMB."
The 43-year-old Brady later joked about the jersey number change with a post on Twitter.
"I would like to speak to the manager please," he wrote.
The NFL's new rule -- which was adopted Wednesday along with multiple other proposals -- unlocks the number of players who are eligible to wear single-digit jersey numbers.
Previously, only quarterbacks, kickers and punters were allowed to wear single-digit numbers. The rule change will allow running backs, tight ends, fullbacks, H-backs and wide receivers to wear numbers 1-49 and 80-89.
Defensive backs can now choose 1-49, while linebackers can use 1-59 and 90-99. Offensive linemen can pick 50-79, and defensive linemen are able to wear 50-79 and 90-99. Quarterbacks, punters and kickers will remain in the 1-19 range.