Prior was examined Wednesday by Yocum in Anaheim but could not have the MRI at that time because of personal business at home in San Diego. He underwent the test in Mesa on Thursday.
O'Neal was encouraged after Yocum's exam.
"For me, I'm very pleased," O'Neal said. "There's extremely bad news that could've come out of it. This is a posterior shoulder strain, and we'll deal with it like you would a hamstring strain -- unfortunately, it's in the shoulder. For us, it's pretty good news."
Prior had halted a bullpen session on Tuesday, complaining of pain in his right shoulder, which prompted the Cubs to send him to Yocum. Before that, the right-hander had not had any problems this spring.
"They're all going through preventative exercises that we go through in the offseason," O'Neal said. "No, this has nothing to do with anything he's done this offseason.
"Mark felt uncomfortable enough with the way he felt on Tuesday that he was not comfortable pushing it beyond that," O'Neal said.
Could this problem have developed over time?
"He had no complaints until he came off the field the other day, and the complaints were significant enough," O'Neal said. "He was telling us he was definitely in pain making some of his throws, and that's when we decided to progress the way we did. I can't answer why it started the other day, and it hadn't prior."
Because Prior was having an MRI arthrogram and dye was injected, he will be restricted from any activity for 48 hours.
"We will progress him as tolerated -- and I know that's a term you don't like hearing," O'Neal said. "We'll let him do what he's capable of doing, we won't try to do more and we'll try to push him as much as we can without jeopardizing any setbacks."
The reason Prior stopped his session was because he was experiencing pain, not soreness. There's a difference.
"There isn't a pitcher out there who we don't treat for soreness," O'Neal said. "Unfortunately, Mark was describing it as pain."
Once again, the Cubs have to deal with Prior and Kerry Wood out with injuries. Wood is rehabbing from arthroscopic shoulder surgery he had last August. He underwent arthroscopic knee surgery last week on his right knee to take care of a tear in his medial meniscus.
"You would think over the course of a winter -- actually three winters, two winters -- that guys would come back strong and healthy, which hasn't been the case," Cubs manager Dusty Baker said. "Woody had a little setback with his knee, but his arm is doing well. He's throwing the ball great. Hopefully, Woody will be ready soon. How soon we don't know. We have Wade Miller coming on pretty strong. We just have to pull our belts a little tighter and do what we have to do."
The Cubs had decided at the end of last season to put Prior on a structured program this spring in an attempt to avoid any injuries. He has opened the two previous seasons on the disabled list because of injuries, first to his Achilles' tendon and last year, because of his elbow.