Gallinari, who tore his ACL in April of last season, did not have ACL surgery at that time, but instead repaired his meniscus, as doctors felt it the ligaments may heal on their own. He was expected to return at the beginning of this season.
"It was recently determined that the procedure that Danilo underwent on his knee this past summer was insufficient," Nuggets general manager Tim Connelly said in a statement. "Danilo's knee required that he undergo reconstruction of the ACL, which was successfully completed earlier this morning.
"Knowing Danilo's drive and work ethic, we look forward to a full recovery and a healthy return to the court next season."
Gallinari, 25, averaged 16.2 points and 5.2 rebounds in 71 games last season.
"It's disappointing," Nuggets coach Brian Shaw said, via the Denver Post. "The fact that we know how hard he worked rehabbing over the last few months to try to get back for this season. We feel for him, and know how tough a situation that is to deal with."