The former NHLer will miss the entire Calder Cup championship series against the Hamilton Bulldogs, which begins Friday in Hershey.
The confrontation involving Brennan and the fan occurred in Hershey's 4-1 road win over Manchester in Game 3 of the Eastern Conference final on Wednesday.
"While it appears that Mr. Brennan was provoked and that the confrontation was initiated by the spectator, Mr. Brennan has violated our rules by responding," AHL president and CEO David Andrews said Monday in a statement.
According to the Lebanon Daily News in Pennsylvania, a Manchester fan attempted to throw a punch at Brennan while he was sitting in the penalty box. The paper said Brennan reached through a space in the glass and tore the fan's shirt.
Security officials grabbed the fan, the Daily News said, while a linesmen stepped into the box to restrain Brennan, who was assessed a gross misconduct penalty.
"We were chirping back and forth, and the fan took it too far," Brennan said after the game. "It happened. I'm glad it got broken up and that no one got hurt."
Hershey coach Bruce Boudreau said the fan was to blame for the incident.
"That guy made a mistake," Boudreau said. "I don't think Kip was doing something. I've got to think that guy initiated the whole thing."
Brennan, 26, was suspended seven games by the ECHL in November after he was involved in a bench-clearing brawl in his first game with the Long Beach Ice Dogs.
The six-foot-four, 222-pounder charged out of the penalty box at the end of a loss to Phoenix and took on several Roadrunner players.
Brennan played parts of four NHL seasons with Los Angeles, Atlanta and Anaheim.