The title had been vacated back in January when Ryo Akaho (31-2-2, 20) [赤穂 亮] was forced to pull out of a bout against Yusuke Suzuki (9-3, 6) [鈴木悠介], in a similar though not identical circumstances. Suzuki was himself injured before a scheduled fight with Muranaka in April.
Now logically, though when does boxing use logic, a match up between Suzuki and Saito would make sense though it's unclear if that's now the option the JBC have in mind.
Sadly for Muranaka this could essentially end his career. He has had weight issues in the past, failing to make weight several times including failing to make the Flyweight limit for a defense of the Japanese Flyweight title. Failing to make weight for that bout really was the start of Muranaka's problems and since then he has suffered two suspensions. It's now hard to see the JBC giving him any more chances, especially given their crack down on fighters failing to make weight this year.
(Image of Saito courtesy of boxmob.jp)