The Atletico Madrid star said: "The competition was hard at Manchester, there was [Ruud] van Nistelrooy, [Dwight] Yorke, [Andy] Cole, [Ryan] Giggs, [Ole Gunnar] Solskjaer.
"I only appeared for 10 minutes every time. That is why I moved to Villarreal, who were not the club they are today.
"I left United to revive my career and this challenge proved a good decision."
He said about his future: "I am not forced to leave Atletico, speculating is useless. I am very happy in there. If something comes, it will be studied."