Prior's agent, John Boggs, confirmed Friday that the pitcher has a tear in his right shoulder capsule. Prior, who had surgery on the shoulder last year, will be kept from throwing for at least two weeks and then be re-evaluated.
"We don't know exactly to what severity," Boggs said, "but obviously something is going on there."
Padres manager Bud Black said before Friday's interleague opener in Seattle that he had not received the results of Prior's examination with Dr. James Andrews on Wednesday. Andrews performed the surgery on Prior's shoulder in April 2007.
Prior felt discomfort in his shoulder at extended spring training in Arizona last week, and was sent back to San Diego for an MRI exam. He then visited Andrews in Birmingham, Ala.
The tear in Prior's shoulder capsule was reported by MLB.com on Friday evening.
Prior, who is 42-29 with a 3.51 ERA in his injury-marred career, last pitched in 2006 when he went 1-6 with a 7.21 ERA in nine games for the Chicago Cubs. He was the second overall pick in the 2001 amateur draft and went 18-6 with a 2.43 ERA in his first full year in the big leagues, finishing third in NL Cy Young Award balloting.
San Diego signed Prior in the offseason with the hope he'd be ready to pitch by June 1.
"He's a battler and is willing to do whatever it takes to get back on the mound," Boggs said.
The Padres did get some good news when catcher Michael Barrett threw from behind the plate Friday for the first time since going on the disabled list.
Barrett went on the disabled list April 8 after spraining a ligament in his right elbow. He had been catching in the bullpen, taking batting practice and keeping up with his conditioning, but was kept from making throws from behind the plate to the bases until Friday.
Black said that if Barrett has no discomfort Saturday, he'll be sent out on a rehab assignment with Triple-A Portland and play at least five games with the Beavers before San Diego will consider bringing Barrett back.
"He feels confident. He's been hitting, he's been running, he's been catching in the bullpen. Conditioning wise, he's fine," Black said. "Just physically and mentally needed to make sure the arm is where it needed to be. He feels good about that."
When Barrett heads out to Portland, he'll be joining pitchers Clay Hensley and Carlos Guevara, both of whom made their final outings at extended spring training in Arizona on Friday and are headed for rehab assignments with the Beavers. Hensley is on the disabled list with a right shoulder strain, while Guevara had a strained groin.
Left-hander Justin Hampson also made his last appearance in Arizona on Friday and will be going to Class-A Lake Elsinore for his rehab.