Greenberg has never given up hope, though, and he learned recently that many people feel the same way. More than 20,000 people have signed an Internet petition to get Greenberg a chance to write a better end to his big league career, and the 31-year-old said Friday that he's touched by the sentiment.
"It's very humbling and very inspiring to have that kind of positivity," Greenberg said of the response. "People can relate to working really hard and having an opportunity to live out their dreams -- or even never getting that opportunity. That's something people can identify with, and it's a part of me. It's a part of my story. If I can touch people for even a few moments of their life, then it's all been worth it."
Greenberg, currently training in an attempt to make Team Israel's submission to the World Baseball Classic qualifying tournament, has never allowed baseball to be far from his thoughts. And though the road has been difficult, he's never stopped believing that he has the ability to make it back.
It all seemed so simple a baseball lifetime ago, when Greenberg was a 24-year-old at the peak of his powers. Greenberg, a ninth-round draftee, had performed well at Double-A West Tennessee in 2005, and he can remember everything about his brief exposure to the Major Leagues.
More than seven years later, Greenberg can cite the weather and tell you what he did in the clubhouse. He remembers manager Dusty Baker tabbing him to pinch-hit in the ninth inning. And he can recall his lone at-bat, a brief encounter against the Marlins' Valerio de los Santos that irrevocably changed his life.
"While it was coming at me, I'm telling myself, 'Stay in.' You're taught not to bail, and you don't want to bail on the first pitch you see in the big leagues,'" Greenberg said. "I didn't black out. I didn't lose consciousness. And after it hit me, you can see me holding my head in the video. I thought I was holding it together. I thought it had split open."
That moment, etched in time, was unforgettable for anyone who witnessed it. Ryan Dempster, a longtime Cub who was traded to the Rangers this season, had no problem recalling it.
"I was the one who called his mom, just to let her know everything was going to be all right. ... I'm sure it wasn't fun to see that as a parent, so I just tried to reassure her that everything is going to be all right," said Dempster. "As teammates we all just tried to rally around him and provide some positive support. It was sad for both guys, him and the pitcher. I'm sure the pitcher wishes it never happened."
For Greenberg, that wish was a constant thought. The outfielder went back to the Minors and tried to play through the after-effects of his concussion, but that never really worked. Greenberg, in fact, said he struggled with his vision and positional vertigo for the better part of two years.
Sometimes, Greenberg said, he would be symptom-free for 72 hours but only because he had spent the time in a recliner wearing a neck brace. Greenberg's symptoms often returned at inopportune moments, which was frustrating for him and for his coaches who didn't know how to get him back in the lineup.
With all that weighing on him, Greenberg said he felt himself slipping into depression.
"It was traumatic," Greenberg said. "I was always upbeat and optimistic, but I became emotionally withdrawn and I didn't care about things. I wasn't suicidal, but I didn't care. I remember being on a plane and thinking, 'If it goes down, who cares?' Years later, I told that to someone, telling him how I had felt and that I'd gotten through it, and he said that, 'Hearing you now gives me hope I can get through it.'"
Greenberg, in time, learned to control his vertigo, but then he endured a much more common baseball injury. Greenberg tore the rotator cuff and labrum in his left shoulder while playing independent-league baseball in 2009, and he needed surgery before the 2010 campaign.
He worked his way back and played two years with the independent Bridgeport Bluefish, but then he took a year to step away from the game and run a business. Greenberg had been playing for $2,200 a month and no health insurance in the independent league, and there came a point when he just couldn't afford it.
And that's where his story took another interesting turn. Filmmaker Matt Liston, a lifelong Cubs fan, had first learned of Greenberg as a prospect, and his curiosity was sparked by the player's odyssey. Over time, Liston kept track of Greenberg's progress in his recovery and quest for redemption.
Last spring, Liston began interviewing people at Spring Training about Greenberg's experience, and he began to notice a common thread in the discourse. Many people had empathy for the situation, and several people took it a step further: They thought something should be done.
"Not only for me, but for Adam, it came at the right time," Liston said of his project. "I didn't want to do anything earlier, because I didn't think the time was appropriate. He could still make it. But you can see he's still in great shape, and at age 30 or 31, he knows that his window is starting to close."
Liston launched his petition -- which calls on the Cubs to give Greenberg one more at-bat -- on Sept. 4. Hall of Famer George Brett has tweeted in favor of it, and it's even made its way through the corridors of big league power.
The Cubs declined to comment about Greenberg for this story, but general manager Jed Hoyer spoke to local reporters recently and tried to put the matter to rest.
"Adam made the big leagues based on merit in 2005," he said. "While it is unfortunate he got hit in his first at-bat, he is in the Baseball Encyclopedia as a Major Leaguer and he should be incredibly proud of that. We wish him the best, but there are no plans to add him to the roster now or in the future."
For Liston, that's only the beginning of the story. He maintains hope, saying that there might be other teams that want to seize the moment while it's still a cause celebre.
Liston said that former Cubs pitcher Kerry Wood e-mailed his site on Monday in support of Greenberg, and former Olympian Janet Evans and current Marlins outfielder Logan Morrison have used their voices to draw attention to the cause. Sooner or later, Liston maintained, some team will heed the call.
Jerome Williams, Greenberg's former teammate with the Cubs and an opponent in independent ball, has his own perspective. Williams, now with the Angels, said that he knows how hard Greenberg worked to come back from the injuries and that he holds him in high regard.
Williams empathized with Greenberg, and he respected him as a ballplayer. And when he's asked if he approved of the campaign to get him one more at-bat, Williams extended conditional support.
"It might be a good idea for him to have another opportunity to be in the big leagues, and to have maybe that one hit that he wants. Or maybe even more," Williams said. "It's up to the organization, and if he proves himself to get to the big leagues, then I think they should give him the opportunity."
And that's all Greenberg wants: a chance. Greenberg hopes to play for Team Israel, and he's up-front about his chance of making the team. He's already been told that he's the 29th player on a 28-man team, but he's looking to buck the odds.
Playing for Team Israel, Greenberg said, would be the joy of a lifetime, and he said that if he doesn't make the team as a player, he wants to stay as an unofficial coach. If it breaks down that way, who could possibly be better to talk to the team about adversity and persistence?
"Mine was an amazing dream come true, the opportunity of a lifetime. And then everything changed," Greenberg said. "Have I ever asked, 'Why me?' Yeah, of course, but it's just like anything beyond my control. I know there's something bigger that I got to be a part of and I still have the burning desire and passion to play baseball. If I can inspire people along the way, that's great."