O’Neill is keen to keep the 20-year-old striker at the club.
“We’ve had a few phone calls from clubs abroad that would like to take Ji on loan, but we’ve knocked them back,” O’Neill told the Sunderland Echo.
“The fact is, we can’t afford to lose any of our squad at the moment, but besides that, Ji is a player who has done well for us.
“The lad has talent, but he’s just come over here and is still learning the game. We want to see how he progresses and develops.
“Apart from that, he’s just scored one of the most important goals that I can remember – that amazing last-minute winner against Man City and making himself an instant hero in the eyes of the fans – and myself – in the process.”
