"Bob is one of the original Bulls greats," Jerry Reinsdorf, Chicago Bulls chairman, said in a statement. "During his nine seasons with the Bulls, Bob became a cornerstone of the franchise, and his tenacious defense set high standards for competition and toughness."

"Toughness was not only a trademark of Bob’s play on the court, but also apparent in his resiliency throughout his basketball career."

Butterbean -- who was a member of the franchise from 1968-1976 -- was selected in the fourth round of the 1965 NBA Draft by the Cincinnati Royals. After spending a year in the Eastern Basketball League, he tried out for the Royals again in 1966.

He then spent two years with the team ... before being selected by the Milwaukee Bucks in the NBA Expansion Draft -- who traded him to the Bulls that same year.

In his nine years with the Bulls, the 6-foot-8 forward averaged 21.3 points per game with 6.8 rebounds per game.

The three-time NBA All-Star called it a career after the 1976-77 season ... and had his jersey retired in 1994.