It's his back this time. :sad:
According to Japanese TV commentator who interviewed Kei's physical trainer, he's been having this problem since a few days before USO.
Cipolla was playing really well, but Kei's FH and BH lacked explosiveness and he wasn't moving well like usual. He had medical time out during the second set but seems it didn't help much.
Hope he'll be fit for DC and the Asian swing.