In his rookie year of 2006, Ryu finished with an 18–6 win-loss record, a 2.23 ERA and 205 strikeouts in 201.2 innings pitched.
In July 2005, Ryu was selected by the Hanwha Eagles as the 1st pick in the second round of the 2006 KBO League Draft, and made his professional debut on April 12, 2006. In the game against Seongnam High School in the quarterfinals, he pitched a shutout, striking out 17. He competed at the 60th National High School Baseball Championship in 2005, when as a third-year student in high school. During the competition, he pitched 8 + 1⁄ 3 innings with 14 strikeouts, and gave up an unearned run on five hits in three games (one start).
Ryu started the semifinal match against Chinese Taipei ( Taiwan), and helped his team reach the final, recording ten strikeouts and giving up one unearned run on four hits in six innings.
In 2005, Ryu was selected for the South Korea national junior team that was runner-up in the 6th Asian Junior Baseball Championship in Seoul, South Korea. In 2005, he led his team to the Blue Dragon Open National High School Championship, pitching 22 consecutive scoreless innings as the team's ace and batting. In 2004, Ryu underwent Tommy John surgery and did not pitch in any official games. When Ryu was 10, his father bought him a lefthander's glove that goes on the right hand, so Ryu learned to pitch with his left hand. He is working towards a master's degree in community physical education in Daejeon University. Ryu was born in Incheon, South Korea on March 25, 1987, and attended Incheon's Dongsan High School.