Gilbert, 26, hurt his left knee in the third quarter of Friday night's 26-point loss to Collingwood at Etihad Stadium.
He planted his leg and twisted awkwardly before being helped from the field by trainers.
There was some conjecture over whether surgery would reveal more damage than was initially considered.
But Gilbert has required just the one repair and will be out for between two and three months.
In other Saints news, veteran midfielder Lenny Hayes will miss between two and four weeks with a calf injury.
The 33-year-old injured his leg in the warm up of the Collingwood game and was a late withdrawal, replaced by Jack Newnes.
Forwards Beau Maister (wrist) and Adam Schneider (hamstring) are inching closer to availability with both slated to return within the next few weeks.
Defender James Gwilt (knee) played in the VFL on the weekend in his first game since an injury in round one.
Jennifer Phelan is a reporter for AFL Media. Follow her on Twitter @AFL_JenPhelan.
