The South Korean midfielder picked up a hamstring injury on the opening day of the season at Chelsea .
Is in a race against time to face the club where midfielder had a successful loan period during the 2013 - 14 season .
The South Korean midfielder picked up a hamstring injury on the opening day of the season at Chelsea and is in a race against time to face the club where he had a successful loan period during the 2013 - 14 season .