The Nuggets offered the update on Monday night via the team's Twitter account, disclosing that Gallinari has torn anterior talofibular and calcaneofibular ligaments and a sprain of the deltoid ligament.
Gallinari suffered the injury in the third quarter of the Nuggets' overtime loss to the Dallas Mavericks on Friday night.
"It's worse than the one I had just two months ago," the 27-year-old said of this injury in comparison to the one that caused him to miss six games in December.
Gallinari had an MRI on Saturday to confirm the sprain and is expected to be out indefinitely. With the Nuggets unlikely to overcome the six-game deficit between them and the eighth playoff spot in the Western Conference, the team could opt to sideline Gallinari for the remaining 22 games to let his ankle heal.
Gallinari is averaging a career-high 19.5 points and 5.3 rebounds in 53 games this season.
ESPN staff writer Tim MacMahon contributed to this report.