On August 1 the band announced that they were canceling three shows after Whibley was diagnosed with a severe case of bronchitis.
Initial news of the attack came early Friday morning on the band's twitter page. "Deryck Whibley was attacked in a bar last night by 3 unknown people. He is in the hospital now. Japanese police are looking into the matter," his bandmates tweeted.
A post to Facebook on Friday evening revealed that Whibley was well enough to play a show scheduled later in the day.
"Deryck Whibley Update: After an MRI scan, it appears Deryck has slipped a disc in his back once again. Despite doctor's instructions, he has decided to play today's show at Summersonic regardless. Sum 41 will perform at 3:40pm," the post read.
Sum 41 is in Japan to perform at the two-day Summer Sonic Festival in Osaka and Tokyo.