On September 13, Ji Soo's label Prain TPC announced,

"Ji Soo received surgery for acute osteomyelitis this September 13. The surgery ended well, and he is currently resting in his hospital room. The surgery came up suddenly, so we are only now delivering the news. 

After some unidentified pain in his legs recently, Ji Soo first sought a hospital last week and went through several check ups and treatments, but showed no signs of improvement, so took a day on September 12 to receive a more specific diagnosis. Even before his diagnosis results came out, on the evening of the 12th, he went to the hospital again for heavy fever and pain, and went in for surgery after the doctor's recommendation at 4PM on the 13th.

What he had was acute osteomyelitis (chronic infection in an open injury to bone or surrounding soft tissue). He is currently recuperating after a successful surgery, but it is likely that we will be able to give more specific details after getting a better explanation from the doctor and after seeing his progress.

We are currently in talk with producers so that his schedules for current drama 'Fantastic' will suffer the least amount effects. But for now, we will put our greatest efforts into helping Ji Soo recover and console his family. We apologize for delivering worrisome news to families ahead of Chuseok." 

We're happy to hear that the surgery was successful. Get well soon!