Baldelli quickly established himself as an excellent hitter and outfielder, and placed third in voting for American League (AL) Rookie of the Year.

He was unable to play at all in 2005 and missed significant time over the following seasons as doctors struggled to diagnose and treat his condition; from 2005 until his retirement in 2010, the former everyday center fielder spent long stretches on the disabled list and only once appeared in more than half of his team's games.

Baldelli signed a free-agent contract with his hometown Boston Red Sox in 2009 and appeared in 62 games as a backup outfielder and part-time designated hitter.

Unsure if he was physically capable of continuing his playing career, he returned to the Rays organization as a minor league coach in December 2009.

He appeared in 10 games over the last month of the regular season and was included on the Rays' playoff roster, but severe muscle cramping during the team's first round series forced him out of the lineup, at which point he decided to end his playing career at age 29.

After retiring as a player, Baldelli spent three years in the Tampa Bay Rays organization as a roving minor league instructor and special assistant to baseball operations.

[6] During his senior year at Bishop Hendricken High School, he pulled his oblique muscle, but still managed to hit .531-5-13 with nine steals in only 32 at-bats.

Baldelli came in third in the voting for 2003 AL Rookie of the Year behind New York Yankees' outfielder Hideki Matsui and winner Ángel Berroa of the Kansas City Royals.

After these setbacks, Baldelli underwent extensive medical testing to determine the reasons for his muscle problems and worsening fatigue after even brief workouts.

On March 12, he held an emotional press conference in which he announced that he would be once again placed on the disabled list as he tried to find an effective treatment for his mysterious ailment.

[24] In Game 2 of the World Series against the Philadelphia Phillies, Baldelli made a double play by catching a fly ball and throwing back to first baseman Carlos Peña in time to beat Jayson Werth.

During the Red Sox series, the United Mitochondrial Disease Foundation created a page on its website where children and parents could post notes for Baldelli.

"[28] During the 2008-09 offseason, further medical testing indicated that Baldelli suffers from a form of mitochondrial channelopathy, which makes his condition less serious and more treatable than previous diagnoses had suggested.

[29][30] Mitochondrial channelopathy is a rare cell disorder that affects ions in neurological "pathways" and causes severe muscle fatigue and can be life-threatening, but Baldelli was diagnosed with a moderate form which can be managed with medication and diet.

[32][33][34] On January 8, 2009, Baldelli signed a one-year deal with the Boston Red Sox,[35] reported to be worth a base of $500,000 plus up to $6.75 million in incentives.

For much of the season, however, he continued to struggle through physical ailments, landing on the 15-day DL twice and sitting out numerous contests with hamstring pulls and other issues.

During spring training 2010, Baldelli returned to the Tampa Bay Rays as a special assistant to observe and coach players in the organization's minor league system on baserunning and outfield defense.

[39] While his continuing fatigue problems and a lingering shoulder issue made it impossible for him to play baseball, he expressed the desire to eventually return to the field with the Rays if his physical condition improved.

[38][40] On July 19, 2010, Baldelli signed a minor league deal with the Rays and joined the Charlotte Stone Crabs, the team's Class A affiliate.

[44] Baldelli played occasionally throughout September, was included on the Rays' postseason roster, and started at DH for the first game of the 2010 playoffs against the Texas Rangers.

However, he suffered muscle cramping during the game and had to be removed for a pinch hitter, and the Rays replaced him on their playoff roster the next day, again putting his playing career in doubt.

[45] Baldelli privately decided to retire soon after his medical condition forced him out of the 2010 ALDS, but Rays' executive Andrew Friedman advised him to take some time to think about the decision before announcing it publicly.

After three months of consideration, Baldelli officially announced his retirement on January 26, 2011, at 29 years old, stating that due to his illness, "I physically don't feel like I should be playing anymore.

[48] Baldelli was a balanced ballplayer who could hit for average and power, had good speed in the field and on the basepaths, and a strong throwing arm.

[8][49][50] After ending his playing career in 2011, Baldelli remained with the Rays organization as a Special Assistant for Baseball Operations and roving minor league hitting and outfield defense instructor.

[62][63] Baldelli is an avid music fan, who names Phish, the Grateful Dead, Greensky Bluegrass, JRAD and Spafford among his favorite groups.