Aiken attended Cathedral Catholic High School in San Diego, California. Aiken pitched for the 18U United States national baseball team in 2013 and helped lead the team to gold in the 2013 18U Baseball World Cup. He started the gold medal game and pitched seven innings with 10 strikeouts and giving up only one run. In February 2014, he was awarded the USA Baseball International Performance of the Year Award for his play. Aiken was considered one of the top prospects heading into the 2014 Major League Baseball draft. In March 2014, he was ranked by Keith Law as the top prospect for the draft. The Houston Astros selected Aiken with the first overall selection. After issues arose with his health, elbow inflammation in his throwing arm, and disputes over an adjusted signing bonus, Aiken was unsigned when the July 18 deadline elapsed. Soon after the draft, it appeared the two sides had agreed to a $6.5 million bonus. But after Aiken's physical, the Astros reduced their offer to $5 million. He was the first number one overall draft pick not to sign since Tim Belcher in 1983. On August 27, it was reported that the Astros could still sign Aiken, as the other 29 MLB teams agreed to allow the use of a clause in the draft which negates the deadline. It was also reported that, if he were to sign, Aiken would insist on a sign-and-trade deal. In February 2015, Aiken enrolled at IMG Academy's postgraduate baseball program, making him eligible for the 2015 Major League Baseball draft and also joining him with Jacob Nix, who was also drafted by and opted not to sign with the Astros in 2014. Aiken was removed in the first inning of his first start for IMG, citing elbow discomfort. On March 26, Aiken announced that he underwent Tommy John surgery after being told he had torn the ulnar collateral ligament in his left elbow.
Professional career
The Cleveland Indians selected Aiken with the 17th overall selection of the 2015 draft. Aiken signed with the Indians agreeing to a signing bonus of $2,513,280. Aiken made his professional debut with the Arizona League Indians in June 2016, and he was later promoted to the Mahoning Valley Scrappers. Aiken ended 2016 with a combined 2–5 record and a 5.83 ERA between both clubs. He spent the 2017 season with the Lake County Captains of the Class AMidwest League, where he posted a 5–13 record with a 4.77 ERA in 27 games. He did not pitch in 2018 and pitched in two games during the 2019 season. Aiken took some time away from the game of baseball afterwards, and the Indians were unsure of his status going forward for the 2020 season.
