The Cubs pitcher made his debut at extended Spring Training on Thursday in Mesa, Ariz., and after throwing two innings, he experienced discomfort in his right shoulder and left the game. Prior had been scheduled to throw 45 pitches, or three innings, whichever came first.

Prior was scheduled to see orthopedic specialist Dr. Lewis Yocum in California for further evaluation, according to a statement issued by the team Thursday.

Prior, 26, appeared in four Cactus League games, making three starts, and totaled 10 1/3 innings. He had felt healthy and was able to keep pace with the other pitchers. But on March 28, the team announced Prior would be optioned to Triple-A Iowa, and not break with the big league team. He stayed in Mesa when the Triple-A team broke camp, and was placed on the Minor League disabled list with right shoulder fatigue.

Prior's plan had been to get on a regular routine. He had problems with his shoulder in 2006 and made nine starts before he was shut down.

"Last year wasn't an easy year for me physically," he said in late March. "It was trying to get side work in. It's been nice to go through [spring] camp and be on a normal schedule. I think they want me to keep doing that."

He even joked that his plan was to try and make the Triple-A All-Star team and Futures Game. An 18-game winner and All-Star in 2003, Prior has been on the disabled list at least once each season since he broke into the big leagues in May 2002.

In his last Cactus League game March 28, Prior threw 60 pitches over three innings.