Tottenham forward Heung-Min Son has said he "felt physically empty" during the Asian Cup after South Korea suffered a shock quarter-final exit.
Despite being considered contenders to win the trophy, South Korea lost 1-0 to Qatar on Friday, leaving Son free to return to London.
With Harry Kane and Dele Alli out injured until March, Son's early return is a huge boost for Mauricio Pochettino, but his latest comments will be of concern to the Spurs manager.
"I wasn't fully prepared," revealed Son, who played three matches in the United Arab Emirates after arriving before South Korea's final group-stage game with China.
"I felt physically empty. I feel so sorry to have let down my team-mates, coaches and the fans.
"I would rather not speak about it really but I haven't felt right physically. I was losing sleep - I should have looked after my condition better."
Son provided two assists in his first game of the tournament as South Korea beat China 2-0 in their final group fixture, but the 26-year-old made little impact during an extra-time victory over Bahrain and the defeat to Qatar.
It was the first time since 2004 that South Korea have been knocked out of the Asian Cup before the semi-finals.
"I thought I would start to feel better and grow into the tournament but it just didn't happen," Son said.
"Many people expected big things of us and I'm annoyed at myself for not being able to step up. I know I didn't perform well."
Boy looks ragged. Ideally he would get the Watford game off to recharge but I can see him be on the bench