The Japanese full-back made a strong start, scoring Saints’ first goal back in the Premier League and assisting the winner against Everton.
However, his form has dipped since, leading to inconsistent game time and two costly errors resulting in goals.
"I'm not satisfied at all with the results or how my performance is," he told the Daily Echo.
"(After the) first 10 games, I felt 'ok, I can play in the Premier League'.
"I felt that I could show who I am with my quality and make an impression. But after that, we were in a difficult situation.
"Then the new manager came and we couldn't win a game. That was not a nice situation for me or the team.
"I'm trying to get better and better, but sometimes I didn't play from the start. It's not good for me, but I'm just trying to keep working hard and trying to be a better player.
"I'm running and training my game every day. And if I don't play the game, I'm just trying to get minutes in the next game. Just every day, step by step.
"But you know, the Premier League is not an easy league. I think it's the best league in the world so I have to adapt."