It has been revealed that although Big Bang's G-Dragon has a cast on his leg, it's not a major injury. On May 16th, a YG Entertainment representative said,

"G-Dragon slightly sprained his leg during the concert. It is not a big injury. He is only wearing the half-cast during performances in case the injury might worsen while moving around. It is okay now. There is no need to worry."

G-Dragon's injury was made known by a tweet from MBC's "Infinity Challenge" producer Kim Tae Ho. He tweeted about Big Bang's Japanese concert, saying, "GD had a light cast on his right foot." While that tweet was made in response to a Big Bang fan, his response quickly informed netizens of G-Dragon's injury. Big Bang is currently on their "Love & Hope Tour" in Japan, hitting up Osaka, Nagoya, and Chiba. Source: My Daily via�Nate