Paparesta and team orthopedist Dr. Will Workman diagnosed Parker with a lateral elbow impingement, but they will wait on an impending MRI to determine the next course of action.
Paparesta is confident Parker will pitch again, but only when he can regain full extension without pain will he begin another throwing program, meaning it's likely impossible he'll pitch in Cactus League games, as was initially planned.
"We're pretty optimistic that things are going to go well, and we're just going to make sure everything is right before we make any more movement toward what we're going to do next," Paparesta said.
The witnessing party to Parker's shortened batting practice session, which included several members of the A's front office and training staff, appeared stunned, in the same way observers reacted when Parker suffered an elbow fracture during a rehab start with Triple-A Nashville in May. But Parker's response this time around was far more encouraging, Paparesta noted.
"He was able to do things he wasn't able to do in Nashville," he said. "After spending some time with him, he was able to move the elbow back and forth and test certain parts of the elbow.
"It did not appear to be as serious as that incident as Nashville. I think the initiation of feeling that extension and that grab is what got his attention, something he hadn't felt yet throughout this entire process."
As described by Workman, "He's been feeling progressively better about his ability to release the ball and extend, and he has had some [inflammation] in the lateral elbow, and the feeling is that he hasn't quite adapted to the workload with the extension. On one of these pitches, it kind of came back to bite him a little bit, so he felt that extension impingement and had a little bit of a muscular recoil as a response to that, and that's obviously quite a shock when you're throwing."
There's no way of identifying such an issue, both said, until an episode like this occurs.
"It's obviously tough," manager Bob Melvin said. "Your initial thoughts are not good, but we'll see. I was encouraged by what I saw afterward."
Parker, 27, has endured an exceptionally difficult career, but he's remained positive throughout, and Paparesta said Thursday the pitcher was understanding of his situation. He hasn't pitched for the A's since 2013, when he went 12-8 with a 3.97 ERA in 32 starts as a key member of the rotation.
The A's have been open to the idea of stretching him out as a starter again this year, but it's unclear whether Parker's latest setback will derail those plans and force him into a bullpen role when healthy.