"It was rough, something I hoped wouldn't happen," Richard told reporters after the Padres defeated the Dodgers, 5-2. "I had some discomfort this week leading up to [the start], but I thought I'd be able to get through. Unfortunately, once I got going, I knew things weren't right."
Richard threw a ball to Dodgers leadoff hitter Mark Ellis and then a strike on his second pitch before leaving the game. The second pitch registered as an 86-mph changeup.
Richard was met near the first-base line by manager Bud Black before he left for the home clubhouse.
Tim Stauffer came in to replace Richard and allowed one run over four innings to earn the victory. Stauffer could be a candidate to replace Richard in the starting rotation, though the Padres won't be in a hurry to make that decision.
Richard entered the game with a 2-5 record and a 7.01 ERA in 11 games, including 10 starts. His previous start before Friday was one of his best this season, as he allowed one run on seven hits in eight innings against the D-backs on Sunday.
"He had a little bit of soreness last week after his start but felt pretty good leading up to this start, where he wanted to give it a go," Black said. "We felt as though he was going to be OK."
Richard had arthroscopic surgery on his left shoulder in July 2011, as team physicians Heinz Hoenecke and Jan Fronek smoothed out the glenoid labrum, biceps tendon and rotator cuff, though Richard said this was a different area altogether.
"It's something totally different [from where his shoulder was surgically repaired]. The AC joint is in a different area," he said. "I think that, once we get it isolated and figure out exactly what's going on, we'll be able to get it calmed down and go from there."
Richard missed 19 games earlier this season on the disabled list with an intestinal virus.
Corey Brock is a reporter for MLB.com. Keep track of @FollowThePadres on Twitter. This story was not subject to the approval of Major League Baseball or its clubs.