The centre-back left after seven and a half years at the club during the January transfer window.
He departed to Sampdoria on loan on deadline day.
He was told about the move while traveling to a game against Liverpool with his teammates.
"Nobody expected me to leave Southampton, the media, teammates, fans or staff," explained the Japanese international to reporters.
"I felt good there, but I played little.
"It was a dilemma.
"In the end I had the courage to try, and the desire to improve further to go further. Instead of being content to sit on the bench and witness the generational change from there.
"I would like to play again and be the protagonist of the matches and my career."