Sasaki was, a few years ago, the top Middleweight in the Land of the Rising son. He had claimed the crown with a 4th round KO win over veteran Tadashi Yuba and put himself at the top of the proverbial domestic tree. Since that win however Sasaki has struggled with form, losing 4 bouts in a row, and he is now win less in close to 4 years. Another loss and it really will be it for him as a relevant name, and he'll likely be forced to seek other ventures away from the ring, especially given 3 of his recent losses have been by stoppage.
As for Shindo this will be his first bout since he lost his title to Toshio Arikawa, courtesy of a 10th round TKO in what was Shindo's first defense. Although better in form than Sasaki we have seen Shindo go 1-2-1 in his last 4 and suffer his first stoppage loss. He likely has more left in the tank than Sasaki but has struggled with consistency at times and knows that a third loss in 5 will make life very difficult to rebuild from, especially given that he's now 30 years old and moving up in weight for this bout, which will take place at 154lbs.
At the moment we have no details on the other bouts of this card, but this really is a must win for both men, with a loss likely being one too many to comeback from.