Still, the outfielder tried to return to the game that he loved and stayed in Boston's farm system in the ensuing years.
Westmoreland served as a designated hitter in some instructional league games in the Dominican Republic in December 2011. However, Westmoreland had a setback with his condition last July and had to have another surgery -- the one that ultimately led to his retirement.
"With a clear mind and heart, as well as the unwavering support and friendship of my family, friends, agent(s), doctors, therapists and the Boston Red Sox, I have decided to voluntarily retire as a professional baseball player," Westmoreland wrote in his e-mail to the Providence Journal, ESPNBoston.com and WEEI.com. "Although it is a very difficult decision for me, it has become clear that the neurological damage caused by the most recent cavernous malformation and surgery leaves me with physical challenges that make it impossible to play the game at such a high level."
The 22-year-old was a standout player at Portsmouth High School in Rhode Island and was selected by the Red Sox in the fifth round of the 2008 First-Year Player Draft.
"Ryan is a remarkable young man," said general manager Ben Cherington. "He was an incredibly talented baseball player, a special talent as a baseball player. We got to know him more as a person after the first incident a few years back and we've come to learn that he's even a more special person.
"Today's decision by him was something that we knew was coming and we had been talking about it. We just couldn't be more impressed by a human being than we are by Ryan in the way he's handled this, the grace he's shown and he's inspired a lot of people."
Red Sox principal owner John Henry tweeted: "Ryan Westmoreland. A very special young man. Throughout this long, tough fight he's handled all of it magnificently. Wishing him the very best."
Red Sox players were saddened by the news. In particular, it hit home for lefty ace Jon Lester, who was 22 when he was diagnosed with a form of cancer called anaplastic large cell lymphoma in 2006.
"That's a tough deal. I can imagine probably right around the same age as me, going through something like that," Lester said. "Just calling it quits, that's got to be tough. I wish him the best of luck. Hopefully he can still pursue something in baseball and be involved. He's a good kid, and just the little bit I got to talk to him, obviously the circumstances where I got to know him were different than him just being a baseball player. That was tough.
"Like I said, wish him and his family nothing but the best and hopefully something will work out for him where he can still pursue his dream of being in the big leagues at some level with a baseball team."
Cherington did not rule out Westmoreland returning to the Red Sox organization, perhaps even in the near future, but he advised Westmoreland to step away for a bit and evaluate his next step.
For starters, Westmoreland would like to get a college education, something the Red Sox could pay for as part of the original contract he signed out of high school.
"I know he wants to go to college -- that's important, no matter where his path leads," Cherington said. "I know that's important and I know it's important to him. But if baseball is something he wants to be a part of his future down the road -- he still has an intellectual capability to help a team -- and for right now, I just think he needs to spend a little time away finding himself, and I know that's what he's committed to."
For a player like Will Middlebrooks, who was drafted by the Red Sox a year ahead of Westmoreland and is 24 years old, the news was tough to take.
"It's a tough day for us as a team and organization, especially for us guys that were close with him, because we know how hard he worked and how good of a baseball player he was and how good of a person he is," Middlebrooks said.
Westmoreland was able to play in just one Minor League season, hitting .296 with seven homers, 35 RBIs and 19 stolen bases in 60 games for Class A Lowell in 2009.
The reason Westmoreland was such an enticing prospect was because of his combination of speed, power and defense. He was considered a classic five-tool player.
"He was just an elite talent," Cherington said. "I got to see him first in high school, in Portsmouth, and then that summer when he was playing [at Lowell]. He got better every time you saw him. There was truly not much he couldn't do on the field.
"And he was a really smart kid, just a good family, from New England, the whole thing. We were really excited to sign him and he showed a lot early in his time with the Red Sox. Like I said, and I told this to Ryan the other day, for some reason, some people don't get dealt the same hand. Some of those hands aren't fair. He got dealt a bad hand.
"But there is a path for him he's going to find and there's going to be a lot of happiness in his future, and I can't imagine anyone else handling this the way he handled it."
In the end, Westmoreland knew that continuing his playing career wasn't realistic.
"In my heart, I know that I have worked as hard as one possibly could to overcome the obstacles presented by this unfortunate series of events," Westmoreland wrote. "It is with that confidence that I am comfortable turning the page, and searching for 'the reason' that this has happened. I believe that there is a plan for me that will utilize my experiences, however painful some may have been, to do something special in my life. It is time for me to find that path and to pursue it with the same focus and effort that I pursued the dream of playing professional baseball."