PHOENIX -- Oakland pitching prospect Michael Ynoa went over the sequence of pitches he was going to throw in his extended spring camp debut against the Giants on Tuesday in Scottsdale, Ariz., a few times in his head during warmups.
"Today was going to be a big day," Ynoa said he thought to himself. "Today was going to be the first of many steps to the big leagues."
Ynoa did take a step Tuesday, just not in the direction he wanted. The right-hander never made out of the bullpen.
Ynoa, one of the most highly regarded prospects in the A's system, felt some tightness in a muscle outside the top of his throwing shoulder while warming up, and he was shut down for 10 days. Ynoa, who was scheduled to pitch one inning against the Giants, said he hopes to pitch in a game a few days after he begins throwing again.
"I was getting warm in the bullpen, and around the 20th pitch, I felt some pain," Ynoa, 20, said. "I rested a little and came back to pitch. Then it started bothering me more. I had to stop. They told me it was just tight and nothing to worry about. I just needed to rest."
The A's signed the 6-foot-7 Dominican to a then-record $4.25 million bonus in 2008, back when he was 16, and have watched him struggle with injuries almost ever since. Ynoa, who has been on the 60-day disabled list three times in his young career, had Tommy John surgery almost two years ago and has experienced the normal ups and downs that come with the procedure. He threw bullpen sessions last winter.
Overall, Ynoa has just nine innings in the Arizona Rookie League on his professional baseball resume.
"I'm going to keep working hard and hoping things will work out," Ynoa said. "I'm not as frustrated as I used to be. I feel like this experience has really made me strong mentally. I know I will ready when my time comes."
Despite the setback, Ynoa still believes 2012 will be a breakout season. The A's probably hope so, too. Ynoa will be eligible for the 2012 Rule 5 Draft if he is not a member of the 40-man roster at the time.
"It's been a real conservative road, and you do all you can to give him chance to heal," A's director of player development Keith Lieppman said. "He is still growing. We just want to make sure he gets the opportunity to see the big picture and see the investment for the long haul. To rush him was not the best idea."
When Ynoa is healthy, Leippman envisions him pitching 75 innings this season. Ynoa could end up pitching as high as Class A Burlington if he proves he is ready for the workload but for now, the club is using extended spring camp as a training ground.
"Any time you invest in high school or young players from any country, there is some risk involved," Lieppman said. "You don't know the end results. You stay optimistic, and in some cases, it will take a little longer. We are optimistic about his growth and where he is at this stage. We are ready to see what he is all about."
The A's will now have to wait a little longer.
"I was really anxious," Ynoa said. "I was prepared but anxious to get on the mound. Maybe that had something to do with me being tight. I don't know."
Jesse Sanchez is a national reporter for MLB.com. Follow him on Twitter @JesseSanchezMLB.