Pitching in a rehab game for Triple-A Nashville on Friday, Parker threw wildly to the plate and collapsed just off the mound. He rolled around in pain and had to be helped off the field.
"You hope for the best, but watching the video, it breaks your heart," A's manager Bob Melvin said of Parker's injury.
X-rays and a CT scan at Vanderbilt University Medical Center on Saturday revealed a fracture of the medial epichondyle bone. It's in the area where the surgery was performed March 24, 2014.
"We don't know how much this sets back his recovery at this time," A's trainer Nick Paparesta said Saturday night. "He's going to go home, and we'll set him up to see a specialist."
Parker, acquired fromArizonabefore the 2012 season, was 12-8 in 2012 and 13-8 in 2013.
Right-handerA.J. Griffin, also coming back from elbow surgery, flew back to the Bay Area to have his ailing right shoulder examined. The extent of either injury has not been determined.
LeftySean Doolittle(strained rotator cuff) is tentatively scheduled for a rehab assignment Monday.
Information from The Associated Press was used in this report.