Gilbert was subbed after hyper-extending his left knee in St Kilda's 26-point loss to the Pies on Friday night.
Despite good news on the ACL front, Gilbert will still have surgery on Tuesday to repair damage to the knee.
He is expected to miss "several weeks" according to the club, with a time frame to be known once the surgery is complete.
In further bad news for the Saints, champion midfielder Lenny Hayes will miss two to three weeks with a minor calf strain.
Hayes was a late withdrawal from the Saints team on Friday night after injuring himself in the warm-up.
