Sum 41 frontman Deryck Whibley was admitted to hospital on August 5 following an attack by three people in a Japanese bar.
Whibley, 30, was drinking with his bandmates, guitarist Tom Thacker, bassist Jason McCaslin and drummer Steve Jocz when the attack took place. Japanese authorities are looking into the matter.
The frontman was rushed to hospital and underwent an MRI scan, which revealed he sustained a slipped disc in his back, as well as cuts and bruising.
Defying doctors orders, Whibley rejoined the band on August 8, for their set at the Summersonic festival in Osaka.