Add to that the emergence of former Derry City star Paddy McCourt and his deputy Niall McGinn as further competition for the wide midfield positions and it would appear that Koki will really need to work his socks off to fight his way into the team.
However young Koki will not be put-off by the apparent mountain between him and a regular starting place at Celtic, indeed its quite the opposite. Mizuno knows the competition for places is tough, but appears confident despite not having Nakamura to mentor him.
"I am determined to become a very good player for Celtic," he said. "This is my ambition and what I work hard to achieve every day. I enjoy life in Scotland and it is special to be playing for such a famous club such as Celtic.
"Yes, things are different this season to last. Shunsuke is no longer here at Celtic and it was a bit strange at first not having him around. I spent a lot of time in his company in Glasgow because we were both from Japan.
"Obviously it was a massive help for me having him here in Scotland at the same time and he was a big influence of me.
"He was also a huge favourite with the Celtic supporters and scored some wonderful goals for the team. Everyone knows how big a star he is back home in Japan and it also raised my profile in my homeland because I was at the same club as him.
"But Shunsuke wanted to move to a new country and take up a new challenge and now he is in Spain. I haven't had much of a chance to catch up with him because he has been busy settling in with Espanyol and I have been busy with Celtic.
"And all the Japanese journalists have moved on to follow him in Spain. They just go where he goes because everyone in Japan wants to know what he is doing.
"However, if I can become a star at Celtic then maybe the journalists will come back and people will want to read about me.
"That is my ambition. I know if I can do well for Celtic then there will be a lot of interest from back home in Japan."
Koki can at least look back to his brief dalliance in the first team as inspiration, indeed he remembers his time in the first team fondly.
He said: "It was fantastic to come into the first-team last season and scoring a goal on my debut was unbelievable. I was pleased with that and also to play at Ibrox against Rangers.
"I have first-team experience and now I just have to work hard to try and get back into the starting line-up under the new manager. So it is down to me."
Whilst Mizuno may not have Naka to assist him in his quest there are a raft of experienced coaches. Working alongside legends such as Danny McGrain and Neil Lennon has improved his game immensely and Koki has the utmost respect for his mentors.
"Neil has played at the highest level," said Mizuno. "He was the captain here for many years at Celtic and won countless trophies as well as played in the Champions League.
"And before that he played in Premier League in England and represented his country, so he is the perfect role model for any young player looking to succeed at Celtic and it is good for me to be working closely with him."
Lets hope we are seeing you in the First Team Hoops sooner rather than later.
Yours in Celtic