But he maintained his self-belief and under the guidance of United boss Sir Alex Ferguson, who signed him for £4million in 2005, has become a consistent performer at the highest level.
"I always wanted to stay in Europe. When I had that hard time (in Holland) I believe I did not show 100%," he explained. "If I had showed 100% and they had booed me maybe I would have gone back to Korea as my quality would not be enough to play in Europe.
"But the manager (Ferguson) is always encouraging me. Maybe he needs me for my work-rate or movement or something.
"I couldn't score a lot but he still believed in me and that helped me and I now score more goals.
"I think I am a better player having been at United. I am more experienced and I have learned a lot from the best players but I believe I am still not showing everything."