Mbenga, who was inactive for Game Four and was on the Mavericks' bench in street clothes, went into the stands to defend Cassandra Johnson, the wife of Dallas Coach Avery Johnson, who was involved in a heated verbal exchange with several Suns' fans in the first quarter.
He violated a league rule prohibiting players, active or not, from going into the stands for any reason.
"Family business," Armstrong said when asked about the conversation by the Dallas Morning News.
The suspension started with Game Five Thursday night.
The report said Mbenga, who has not played in the series, will lose $48,000 of his $880,000 yearly salary.
NBA senior vice president of basketball operations Stu Jackson said the league will not take action against Mavericks' owner Mark Cuban for his involvement in the incident.