According to WWE, the injury occurred during the Six-Pack Challenge match on last week's RAW.
"Jeff is dealing with a rotator cuff tear right now," said WWE Senior Ringside Physician Dr. Stephen Daquino. "It's a full thickness tear of his supraspinatus tendon and a couple of other areas in the shoulder."
Dave Meltzer reported on the Wrestling Observer Radio that the injury actually occurred during the 30 minute Iron Man match at the Great Balls of Fire pay-per-view in July and he's been working with it since then.
Hardy said that his rotator cuff will definitely be repaired, and added that he may have dislocated his shoulder and also tore his labrum during the match.
"Once they get in there, they'll figure out more about what they have to repair," said Hardy. "But for sure, the rotator cuff has to be repaired, and then they'll look at the labrum when they get in there."
Erick Rowan and Shelton Benjamin both recently had surgery for a torn rotator cuff and were out of action for seven months. Assuming a similar recovery time, with Hardy undergoing surgery in early October, that would put his return at around early May of 2018, after WrestleMania 34.