Koo Ja-cheol

Jeju United

In 2007, Koo was selected by Jeju United in the K League draft. He failed to make an impression in his first two years at Jeju due to numerous injuries. However, he slowly broke into the first team as a defensive midfielder. In January 2010, Koo was reportedly invited by Blackburn Rovers for a trial, but the move did not happen. That year, he spent a great season at Jeju instead, driving his club to an unprecedented runner-up finish. After the contribution, he received some individual awards from the league's federation, namely the Top Assist Provider Award and the FANtastic Player Award, as well as being selected for the Best XI.

VfL Wolfsburg

On 30 January 2011, Koo moved to Bundesliga club VfL Wolfsburg, signing a three-and-a-half-year contract for an undisclosed fee. On 12 February 2011, Koo made his Wolfsburg debut, coming on as a 64th-minute substitute in a 1–0 home defeat to Hamburger SV. However, he had difficulty competing for a starting position with his teammates during a year.

Loan to FC Augsburg

On 31 January 2012, he moved on loan to another Bundesliga club FC Augsburg. On 18 February, he scored the equaliser outside the penalty area in a 4–1 defeat to Bayer Leverkusen. On 17 March, Augsburg defeated Mainz 05 2–1 after he equalised with a lob outside the penalty area. On 24 March, he provided an assist for the equaliser in a 1–1 draw with Werder Bremen. On 31 March, he scored the opening goal with a low shot from…

Koo showed his best performance when playing as an attacking midfielder, although he started his professional career as a defensive midfielder. He could play as a central midfielder, second striker, or winger if necessary. He was praised for his techniques, especially a talent for getting out of opponents' pressure with the ball. He also had an ability to find the back of the net, often displaying a knack to charge into the opposition penalty box unsighted. However, his individual ability was occasionally criticised for slowing his team's tempo.

Koo made his senior international debut on 17 February 2008, coming on as a substitute in South Korea's 3–2 victory over China at the 2008 EAFF Championship. The following year, he captained South Korea to the quarter-finals of the 2009 FIFA U-20 World Cup, scoring from the penalty spot in a 3–0 group-stage victory over the United States. Koo attended South Korea's final training camp before the 2010 FIFA World Cup, but one of the last three players to be cut. Later that year, Koo captained the under-23 team to bronze at the Asian Games, scoring three goals, including one in a 4–3 comeback victory over Iran in the bronze-medal match. At the 2011 AFC Asian Cup, coach Cho Kwang-rae moved Koo from a deeper midfield position into a withdrawn-forward role behind Ji Dong-won after Park Chu-young withdrew through injury, a change dubbed the "Koo shift" by the Korean press. He played all six matches and scored four times in the group stage. After having South Korea's first attempt saved in the semi-final penalty shoot-out defeat by Japan, he scored and assisted Ji in a 3–2 victory over Uzbekistan in the third-place match. His five goals earned him the tournament's Golden Boot as South Korea finished third. Koo again served as captain at the 2012 Olympic tournament, appearing in all six matches. In the bronze-medal match against Japan, he was booked for a strong tackle from behind on Yūki Ōtsu and repeatedly demanded "Why? Why? Why?" of the referee. Koo later linked his emotions…

Koo started football at age ten when a nearby elementary school opened its football club. During his early youth career, he mostly played as a defender, often taking the role of sweeper. While playing for Boin High School, he suffered from anemia, but contributed to the school's runner-up finish at the 2016 edition of the Baekrok High School Football Tournament, annually held in Jeju Island. At the competition, he grabbed the attention of Jung Hae-seong, then manager of Jeju United.