"Everything went well," said manager Bob Melvin, who had yet to speak with the pitcher.
Parker has tweeted twice since the procedure, on Monday telling his followers, "All good! #BeenThereDoneThat lets go Athletics" and on Tuesday, sharing, "Appreciate & want to thank everyone for the support, nice messages and kind words! Finished 1st follow up/therapy."
Parker, 25, was on the heels of a strong 2013 season (12-8, 3.97 ERA) and in line for his first career Opening Day start before learning he had again torn the ulnar collateral ligament in his elbow. He first had the procedure in 2009.
Jane Lee is a reporter for MLB.com. Read her blog, Major Lee-ague, and follow her on Twitter @JaneMLB. This story was not subject to the approval of Major League Baseball or its clubs.