"I don't know what's going on inside there, but somebody made a mistake," he said, per Jarrett Bell of USA Today.
"I think when you look at the whole situation, you try to figure out how you want to get away from things that are there," he continued. "I had a different story, where they had made a decision. He, obviously, they never would have gotten rid of. I still don't understand how New England let him get away. I don't understand that."
Few people can relate with Brady as much as Montana.
The Hall of Famer won four Super Bowls and cemented himself as a legendary figure with the San Francisco 49ers before the team traded him to the Kansas City Chiefs near the end of his career.
He led the Chiefs to the playoffs in each of his two seasons with the team but was unable to lift the Lombardi Trophy.
"I don't know exactly what he's looking for, but my understanding was that he's just looking for more control of the offense," Montana said of Brady. "But I don't know. I haven't had a long conversation with him; I talked to him a little bit at the Super Bowl, but not enough time to really get in-depth."
Montana also suggested Brady wanted more offensive weapons, which he will have plenty of with the Tampa Bay Buccaneers.
Mike Evans and Chris Godwin represent one of the best wide receiver tandems in the league, Cameron Brate and O.J. Howard are capable playmakers at tight end, and running back Ronald Jones II impressed down the stretch last season.
Brady, 42, doesn't have enough time remaining in his career to replicate what he did in New England with six championships, three league MVPs and 14 Pro Bowl selections, but he represents a chance for a Buccaneers franchise that hasn't won a playoff game since the 2002 campaign to turn things around.
New England moved on despite Montana's disagreement, but Tampa Bay has the chance to impress with a new leader under center.