Martinez said of the Japanese winger: "He's been a real surprise. He came in the first day with a big smile and he's never stopped smiling.
"I couldn't work with him enough to get him involved (against Chelsea). The way we play, you need to understand certain concepts, but he's worked twice with the team, he's connected with everyone, and he's an exciting talent.
"We are all really looking forward to seeing what he can do this season."