Ji spent the second half of last season on loan at Augsburg and scored five goals in 17 Bundesliga appearances.
The 22-year-old's impressive form led to a £5m bid from Borussia Dortmund in the summer, but former boss Paolo Di Canio rejected the offer.
Ji has since made just one Premier League start this season, sparking reports that he could leave Wearside in the New Year.
Augsburg are one of a number of sides to be linked with the South Korea international, but Reuter is reluctant to conduct mid-season business.
"It is no secret that Ji had a great season playing for Augsburg. But I do not want to talk so much about new players," Reuter told the Sunderland Echo.
"Basically, we are keeping our eyes and ears open, but I'm not a fan of transfers during the winter break."
Ji's current contract at Sunderland is due to expire in June 2014.