The setback is the second of its kind in as many years for the 27-year-old, who missed the end of last season and start of this year after suffering an ACL setback in 2022.
The club confirmed on Tuesday that Weller will enter another phase of rehabilitation on his knee after suffering a setback late into last Saturday's win over St Kilda.
“Lachie had an incident with his knee in the late stages of the Round 17 match against Port Adelaide,” Suns football and performance boss Wayne Campbell said in a club injury update.
“After subsequent scans and consultations, the decision has been made for Lachie to undergo a second knee reconstruction.
“It's bitterly disappointing news for Lachie and it's important now that, as a club, we provide him all the support he needs.”
Weller joined the Suns ahead of the 2018 season through a trade from Fremantle, where he played 47 matches in three seasons. The Queensland native has since played in 89 games for the Suns, including nine in 2023.
He averaged 21.56 disposals, 6.78 marks and 4.78 rebound 50s per game so far this year.
Weller joins Joel Jeffrey on the Suns' injury list, with the forward currently requiring another four weeks on the sidelines with a foot injury.