The AC joint is located at the junction of the shoulder and clavicle.
The good news is that an MRI showed no damage to the AC joint in Richard's shoulder. Given the timing of the surgery, though, there wouldn't be enough time for rest and rehabilitation in time for him to return during the regular season.
"It's never good to have surgery, but it's nice that it's not necessarily in the shoulder itself," Richard said.
Richard started a game June 21 against the Dodgers but left after throwing two pitches. He's been on the disabled list ever since. At the time, his injury was termed a left shoulder strain.
The team will likely place Richard on the 60-day disabled list Friday to make room for Friday's starter, Sean O'Sullivan, who was with Triple-A Tucson.
As for Richard, he will finish the year with a 2-5 record and a 7.01 ERA in 12 games, including 11 starts.
Corey Brock is a reporter for MLB.com. Keep track of @FollowThePadres on Twitter. Jamal Collier is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.