Tanaka (29) has played two seasons for Otago and the Highlanders but will head back to Japan at the end of the Super 15 to get ready for his season with the Panasonic Wild Knights.
Tanaka has been a revelation for Otago since he arrived here in 2012.
His sharp pass and ability to get to the breakdown and move the ball on was a key part of the quick, attacking play displayed by Otago over the past two years.
Tanaka was the first Japanese player to play Super rugby for a New Zealand franchise but has only had limited appearances because of the presence of All Black halfback Aaron Smith.
With Tanaka out of the picture, Otago can look at a few options behind the scrum.
New Zealand Colts halfback Josh Renton played for Otago last year and there are some other promising No 9s in the club competition.
One possible candidate is Harbour halfback Nick Ealey.
Ealey plays for the same team as Tanaka in Japan. He was born in Christchurch and was educated at St Bede's College, but he went to Japan after school and has played top-class Japanese rugby for some years, filling in many times when Tanaka is not available.
The Wild Knights are now coached by Robbie Deans, who has former Otago coach Phil Mooney as his assistant.
Other possible halfback options include Southern halfback Josh Walden and Alhambra-Union halfback Caleb Gray. Highlanders third-choice halfback Frae Wilson is not contracted to his old province, Wellington, and could be another candidate.
Meanwhile, two Otago forwards who appeared to have had their seasons cut short before they even got started may not yet be off the paddock.
Otago assistant coach Phil Young said lock Jackson Hemopo injured his shoulder in a club game against Zingari-Richmond and the initial fear was that the seriousness of the injury would rule him out for the season.
But Hemopo was waiting to get a second opinion and the injury may not be as bad as first feared.
Highlanders prop Craig Millar had also injured his shoulder when coming on as a substitute in South Africa, and there were concerns he would miss the rest of the season.
But the injury had improved and he was expected to be available some time next month.
