Weller, who joined the Suns in 2018 after three years with Fremantle, has played 11 games this season, averaging 20.5 disposals and 526.8 metres gained per game.
The 26-year-old joins fellow Sun Ben King on the sidelines with an ACL, after the gun forward suffered the injury during the pre-season.
Gold Coast football manager Wayne Campbell confirmed the extent of Weller's injury, with the Sun now set to miss the rest of the season.
"It's really unfortunate news for Lachie," Campbell told club media.
"(He) went in for scans (on Monday) afternoon to determine the full extent of the injury, with the results showing he has torn the ACL in his right knee.
"Lachie will now meet with a surgeon later in the week before having an operation to repair the ligament."
The Suns are currently sitting 11th on the ladder, with their prospect of a debut finals appearance begining to increase. With a 6-6 win-loss record, the Suns still have matches against West Coast, North Melbourne and Essendon to come in the second half of the season.