He also received professional help, which eventually helped bring him back to baseball.
At a media briefing arranged by the Royals following Friday's workout, the 23-year-old old sat behind a rectangular table, several microphones in front him, and spent 32 minutes talking about the circumstances surrounding his unexpected disappearance a year earlier, and what made him return to the game he is so good at.
"The toughest part is convincing people that it's not a problem anymore," he said. "When I left, I was leaving and never coming back. It was one of the best moments of my life to get that off my shoulders. I knew how much I hated baseball at the time and I had no intentions of coming back, at least [last] year. I told myself that there was no way I was coming back, and at the earliest, maybe the next year.
"Anything after that would be too late for me, anyway. I was looking at colleges for me to go to, schools in vacation places like Hawaii, just to get away from everyone I knew, so I could start over. I also thought about mowing lawns for a living because I like to be outdoors.
"But when I was talking to the [psychologist], I kind of realized it wasn't baseball that I didn't like."
He didn't like all the idle time being spent on a baseball field when he wasn't pitching, nor did he understand many of the rules players were expected to follow.
He also was diagnosed with social anxiety.
"It bothered me to be around people, but rules bothered me more than most people," he said. "When I was growing up, I would go to the field and play. It was like that in rookie ball. You went to the field, played catch, did your PFP (pitchers' fielding practice), shag balls and leave. I would get all my work in, and then do whatever I wanted to do. I always thought that's the way baseball was."
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But as he advanced through the Royals' Minor League system, the workouts became more structured and he seemed to spend more time standing around than playing. Greinke said he developed a strong dislike for the game and wished he had been drafted as a hitter because there were more activities, like taking batting practice, fielding ground balls and running the bases.
"You can only throw so many pitches," he said.
The demons that drove Greinke from the game apparently have been conquered and the Royals organization has been supportive throughout the entire ordeal, reactivating him last June and assigning him to Double-A Wichita, where he went 8-3 with a 4.34 ERA in 17 starts.
After the team was eliminated in the playoffs, Greinke received a promotion and made two relief appearances -- both against the Tigers -- and received his ninth MLB win after tossing three scoreless innings.
"When I first left, I didn't think they would [be supportive], but the way they treated me has been great," he said. "This Spring Training has been the best I could have imagined."
General manager Dayton Moore said Greinke's troubled past would not be a factor this spring when it comes to determining the Opening Day roster.
"We have a new group of people here and we're learning all of our players, a lot of them for the first time," Moore said. "My experiences with Zack have always been very positive, very respectful. Certainly, he has done everything he's needed to do.
"I think the great thing about Zack's situation is he has turned it into a positive, which is good to see. We all learn from circumstances that we go through."
"He's smiling more this year," McClure said. "It looks like he's having fun, which is a really nice thing to see. Playing baseball is supposed to be fun."
So far, camp has gone well for Greinke, who says he doesn't even think about his past, and Friday was the last time he would publicly talk about his camp walkout.
His current mission is to prove to the Royals that he belongs in the Major Leagues as one of the team's five Opening Day starters.
"He has to perform in Spring Training," manager Buddy Bell said. "There's no question that he has the ability and will be a big-league pitcher again. But it's up to him to show us that and I think that's great for Zack. I believe it's the way he wanted it all along."
Greinke, who reported to camp a year ago as a strong candidate to be the team's Opening Day starter, believes he can earn a spot in the rotation.
"Even when I was 19, I thought I was the best pitcher on the team," he said, "and if I am the best pitcher, I should be on the team. I feel like I am good enough to pitch here and I have proven it, too.
"I still feel like I am one of the best pitchers on the team and if that's the case, why wouldn't I be on the team? If, for some reason I don't pitch well enough to be on the team, then I shouldn't be on the team. But I don't see any reason why I shouldn't be good enough."
And that part of his story begins the first week of Cactus League games. Greinke is tentatively scheduled to pitch his first two innings of the spring on March 5 against the Padres at Surprise Stadium.