Ji, who moved to Wearside in the summer, was hauled off at half-time in his country's 2-0 weekend win over the United Arab Emirates.
The 20-year-old played 85 minutes against Manchester United earlier this month with no apparent problems but national head coach Cho Kwang-Rae has criticised his fitness levels.
"Ji's condition was really bad," said Cho.
"He was not good in training so I considered not to play him and he was worse in the match than I thought. So I had to replace him at half-time.
"His balance is off and his shooting timing is slow. I think he's physically really poor now."